Split flightgear::initApp() to make it usable even without FGGlobals initialized

- Add an optional argument to flightgear::initApp(): doInitQSettings.
  This argument defaults to true, preserving initApp()'s behavior in
  this respect. If this argument is set to false, FGGlobals doesn't have
  to be initialized.

- New function flightgear::initQSettings(), called by
  flightgear::initApp() when its 'doInitQSettings' argument is true.
  This allows initializing the QSettings exactly when it is needed.

- New function flightgear::checkKeyboardModifiersForSettingFGRoot().
  The code it contains used to be run from initApp(), which is
  undesirable because:

    1) initApp() is not only called at FG initialization (fgMainInit()),
       but also from QtMessageBox(), from QtFileDialog::exec() and twice
       from Options::setupRoot(). However, checking the Alt and Shift
       modifiers to set 'fg-root' in QSettings to the special value
       "!ask" only makes sense in fgMainInit(), not in these other
       places.

    2) This code relies on the QSettings to be set up, and therefore on
       FGGlobals. Thus, freeing initApp() of its dependency on FGGlobals
       requires splitting this keyboard modifiers checking code out of
       initApp().

src/GUI/QtFileDialog.cxx

@@ -40,9 +40,12 @@ void QtFileDialog::exec()
     int fakeargc = 1;
     static char fakeargv0[] = "fgfs";
     static char * fakeargv[2] = {fakeargv0, 0};
-    // This does nothing if it has already been run, so the fake argc/argv
+    // This does nothing if it has already been run, so the fake argc/argv are
-    // are only used if run without launcher
+    // only used if run without launcher. Don't attempt to initialize the
-    flightgear::initApp(fakeargc, fakeargv);
+    // QSettings, because this would require FGGlobals to be initialized (for
+    // globals->get_fg_home()), which would prevent using this function at
+    // early startup.
+    flightgear::initApp(fakeargc, fakeargv, false /* doInitQSettings */);
     
     // concatenate filter patterns, as Qt uses a single string
     std::string filter="";

src/GUI/QtLauncher.cxx

@@ -83,6 +83,7 @@
 
 using namespace flightgear;
 using namespace simgear::pkg;
+using std::string;
 
 const int MAX_RECENT_AIRCRAFT = 20;
 const int DEFAULT_MP_PORT = 5000;
@@ -505,7 +506,9 @@ static void initQtResources()
 namespace flightgear
 {
 
-void initApp(int& argc, char** argv)
+// Only requires FGGlobals to be initialized if 'doInitQSettings' is true.
+// Safe to call several times.
+void initApp(int& argc, char** argv, bool doInitQSettings)
 {
     static bool qtInitDone = false;
     static int s_argc;
@@ -525,22 +528,45 @@ void initApp(int& argc, char** argv)
         app->setApplicationName("FlightGear");
         app->setOrganizationDomain("flightgear.org");
 
-        QSettings::setDefaultFormat(QSettings::IniFormat);
-        QSettings::setPath(QSettings::IniFormat, QSettings::UserScope,
-                           QString::fromStdString(globals->get_fg_home().utf8Str()));
-
         // reset numeric / collation locales as described at:
         // http://doc.qt.io/qt-5/qcoreapplication.html#details
         ::setlocale(LC_NUMERIC, "C");
         ::setlocale(LC_COLLATE, "C");
+    }
 
-        Qt::KeyboardModifiers mods = app->queryKeyboardModifiers();
+    if (doInitQSettings) {
+        initQSettings();
+    }
+}
+
+// Requires FGGlobals to be initialized. Safe to call several times.
+void initQSettings()
+{
+    static bool qSettingsInitDone = false;
+
+    if (!qSettingsInitDone) {
+        qSettingsInitDone = true;
+        string fgHome = globals->get_fg_home().utf8Str();
+
+        QSettings::setDefaultFormat(QSettings::IniFormat);
+        QSettings::setPath(QSettings::IniFormat, QSettings::UserScope,
+                           QString::fromStdString(fgHome));
+    }
+}
+
+bool checkKeyboardModifiersForSettingFGRoot()
+{
+    initQSettings();
+
+    Qt::KeyboardModifiers mods = qApp->queryKeyboardModifiers();
     if (mods & (Qt::AltModifier | Qt::ShiftModifier)) {
         qWarning() << "Alt/shift pressed during launch";
         QSettings settings;
         settings.setValue("fg-root", "!ask");
+        return true;
     }
-    }
+
+    return false;
 }
 
 bool runLauncherDialog()

src/GUI/QtLauncher.hxx

@@ -23,7 +23,15 @@
 
 namespace flightgear
 {
-  void initApp(int& argc, char** argv);
+  // Only requires FGGlobals to be initialized if 'doInitQSettings' is true.
+  // Safe to call several times.
+  void initApp(int& argc, char** argv, bool doInitQSettings = true);
+  // Requires FGGlobals to be initialized. Safe to call several times.
+  void initQSettings();
+
+  // Set 'fg-root' in QSettings to the special value "!ask" if either Alt or
+  // Shift is pressed. Return true in this case, false otherwise.
+  bool checkKeyboardModifiersForSettingFGRoot();
 
   bool runLauncherDialog();
 

src/GUI/QtMessageBox.cxx

@@ -34,9 +34,12 @@ QtMessageBox(const std::string& caption,
     int fakeargc = 1;
     static char fakeargv0[] = "fgfs";
     static char * fakeargv[2] = {fakeargv0, 0};
-    // This does nothing if it has already been run, so the fake argc/argv
+    // This does nothing if it has already been run, so the fake argc/argv are
-    // are only used if an error box is triggered in early startup
+    // only used if an error box is triggered in early startup. Don't attempt
-    flightgear::initApp(fakeargc, fakeargv);
+    // to initialize the QSettings, because this would require FGGlobals to be
+    // initialized (for globals->get_fg_home()), which would prevent using
+    // this function at early startup.
+    flightgear::initApp(fakeargc, fakeargv, false /* doInitQSettings */);
     QMessageBox msgBox;
     msgBox.setWindowTitle(QString::fromStdString(caption));
     msgBox.setText(QString::fromStdString(msg));

src/Main/main.cxx

@@ -495,6 +495,8 @@ int fgMainInit( int argc, char **argv )
 #if defined(HAVE_QT)
     if (showLauncher) {
         flightgear::initApp(argc, argv);
+        flightgear::checkKeyboardModifiersForSettingFGRoot();
+
         if (!flightgear::runLauncherDialog()) {
             return EXIT_SUCCESS;
         }