Integrate the EmbeddedResourceManager into FlightGear

${CMAKE_SOURCE_DIR}/src/EmbeddedResources/FlightGear-resources.xml
(currently empty) is automatically "compiled" into
${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.[ch]xx by
fgrcc inside the build directory. These files are incorporated into the
FlightGear build (FlightGear-resources.cxx is linked into FlightGear).

When the XML embedded resource declaration file added here,
FlightGear-resources.xml, is compiled, fgrcc is passed the
--root=${CMAKE_SOURCE_DIR} option, so that files referred to in
FlightGear-resources.xml are looked up relatively to the root directory
of the FlightGear repository. One could use a second XML embedded
resource declaration file compiled with a different --root option to
grab files from FGData, for instance. I would name such a file
FGData-resources.xml to be consistent with the current naming scheme.

Note: this --root option applies to the paths of real files. Don't
      confuse it with the 'prefix' attribute of <qresource> elements
      inside XML resource declaration files (such as
      FlightGear-resources.xml), which applies to the virtual path of
      each resource defined beneath.

The commands in src/Main/CMakeLists.txt ensure that
FlightGear-resources.xml is recompiled with fgrcc whenever it is
changed, and obviously also when FlightGear-resources.cxx or
FlightGear-resources.hxx is missing. However, CMake doesn't know how to
parse fgrcc XML resource declaration files, therefore when a resource is
modified but the XML file it is declared in is not (here,
FlightGear-resources.xml), you have to trigger yourself a recompilation
of the XML resource declaration file to see the new resource contents
inside FlightGear. The easiest ways to do so are:
  - either update the timestamp of the XML resource declaration file;
  - or remove one or both of the generated files
    (FlightGear-resources.cxx and FlightGear-resources.hxx here).

The EmbeddedResourceManager is created in fgMainInit() just after
Options::processOptions() set the language that was either requested by
the user or obtained from the system (locales). Resources from
FlightGear-resources.cxx are added to it, after which
EmbeddedResourceManager::selectLocale() is called with the user's
preferred locale (obtained with FGLocale::getPreferredLanguage()).

Upon reset (fgStartNewReset()), EmbeddedResourceManager::selectLocale()
is called in a similar way after Options::processOptions(), however in
this case the EmbeddedResourceManager instance doesn't have to be
recreated.

src/EmbeddedResources/FlightGear-resources.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Superset of the resource file format used by Qt.
+     cf. http://doc.qt.io/qt-5/resources.html and 'fgrcc dash-dash-help'
+     (I can't put a double dash inside an XML comment...) -->
+
+<FGRCC version="1.0">
+</FGRCC>

src/Main/CMakeLists.txt

@@ -22,6 +22,7 @@ set(SOURCES
     subsystemFactory.cxx
     screensaver_control.cxx
 	${RESOURCE_FILE}
+	${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.cxx
 	)
 
 set(HEADERS
@@ -40,6 +41,15 @@ set(HEADERS
     subsystemFactory.hxx
     AircraftDirVisitorBase.hxx
     screensaver_control.hxx
+    ${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.hxx
+	)
+
+add_custom_command(
+  OUTPUT ${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.cxx
+         ${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.hxx
+  COMMAND fgrcc --root=${CMAKE_SOURCE_DIR} --output-cpp-file=${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.cxx --init-func-name=initFlightGearEmbeddedResources --output-header-file=${CMAKE_BINARY_DIR}/src/EmbeddedResources/FlightGear-resources.hxx --output-header-identifier=_FG_FLIGHTGEAR_EMBEDDED_RESOURCES ${CMAKE_SOURCE_DIR}/src/EmbeddedResources/FlightGear-resources.xml
+  DEPENDS
+    fgrcc ${CMAKE_SOURCE_DIR}/src/EmbeddedResources/FlightGear-resources.xml
   )
 
 get_property(FG_SOURCES GLOBAL PROPERTY FG_SOURCES)

src/Main/fg_init.cxx

@@ -60,6 +60,7 @@
 #include <simgear/misc/sg_dir.hxx>
 #include <simgear/io/iostreams/sgstream.hxx>
 #include <simgear/misc/strutils.hxx>
+#include <simgear/embedded_resources/EmbeddedResourceManager.hxx>
 #include <simgear/props/props_io.hxx>
 #include <simgear/scene/tsync/terrasync.hxx>
 
@@ -107,6 +108,7 @@
 #include <AIModel/submodel.hxx>
 #include <AIModel/AIManager.hxx>
 #include <AIModel/performancedb.hxx>
+#include <Main/locale.hxx>
 #include <Navaids/navdb.hxx>
 #include <Navaids/navlist.hxx>
 #include <Scenery/scenery.hxx>
@@ -1118,6 +1120,13 @@ void fgStartNewReset()
     
     flightgear::Options::sharedInstance()->processOptions();
 
+    const auto& resMgr = simgear::EmbeddedResourceManager::instance();
+    // The language was (re)set in processOptions()
+    const string locale = globals->get_locale()->getPreferredLanguage();
+    resMgr->selectLocale(locale);
+    SG_LOG(SG_GENERAL, SG_INFO,
+           "EmbeddedResourceManager: selected locale '" << locale << "'");
+
     // PRESERVED properties over-write state from options, intentionally
     if ( copyProperties(preserved, globals->get_props()) ) {
         SG_LOG( SG_GENERAL, SG_INFO, "Preserved state restored successfully" );

src/Main/main.cxx

@@ -56,6 +56,7 @@ extern bool global_crashRptEnabled;
 #include <simgear/math/sg_random.h>
 #include <simgear/misc/strutils.hxx>
 
+#include <Main/locale.hxx>
 #include <Model/panelnode.hxx>
 #include <Scenery/scenery.hxx>
 #include <Sound/soundmanager.hxx>
@@ -80,6 +81,9 @@ extern bool global_crashRptEnabled;
 #include "subsystemFactory.hxx"
 #include "options.hxx"
 
+#include <simgear/embedded_resources/EmbeddedResourceManager.hxx>
+#include <EmbeddedResources/FlightGear-resources.hxx>
+
 #if defined(HAVE_QT)
 #include <GUI/QtLauncher.hxx>
 #endif
@@ -537,6 +541,15 @@ int fgMainInit( int argc, char **argv )
         return EXIT_SUCCESS;
     }
 
+    const auto& resMgr = simgear::EmbeddedResourceManager::createInstance();
+    initFlightGearEmbeddedResources();
+    // The language was set in processOptions()
+    const std::string locale = globals->get_locale()->getPreferredLanguage();
+    // Must always be done after all resources have been added to 'resMgr'
+    resMgr->selectLocale(locale);
+    SG_LOG(SG_GENERAL, SG_INFO,
+           "EmbeddedResourceManager: selected locale '" << locale << "'");
+
     // Initialize the Window/Graphics environment.
     fgOSInit(&argc, argv);
     _bootstrap_OSInit++;