GUI ‘restore defaults’ support.

- Hold ‘alt’ on launch to restore defaults, including launcher prefs.

src/GUI/QtLauncher.cxx

@@ -81,7 +81,7 @@ const int AircraftVariantDescriptionRole = Qt::UserRole + 200;
 
 void initNavCache()
 {
-    NavDataCache* cache = NavDataCache::instance();
+    NavDataCache* cache = NavDataCache::createInstance();
     if (cache->isRebuildRequired()) {
         QProgressDialog rebuildProgress("Initialising navigation data, this may take several minutes",
                                        QString() /* cancel text */,
@@ -1102,6 +1102,18 @@ void QtLauncher::initApp(int argc, char** argv)
         // avoid double Apple menu and other weirdness if both Qt and OSG
         // try to initialise various Cocoa structures.
         flightgear::WindowBuilder::setPoseAsStandaloneApp(false);
+
+        Qt::KeyboardModifiers mods = app->queryKeyboardModifiers();
+        if (mods & Qt::AltModifier) {
+            qWarning() << "Alt pressed during launch";
+
+            // wipe out our settings
+            QSettings settings;
+            settings.clear();
+
+
+            Options::sharedInstance()->addOption("restore-defaults", "");
+        }
     }
 }
 

src/Main/fg_init.cxx

@@ -592,8 +592,13 @@ fgInitNav ()
 {
     flightgear::NavDataCache* cache = flightgear::NavDataCache::instance();
     static bool doingRebuild = false;
-  if (doingRebuild || cache->isRebuildRequired()) {
-    doingRebuild = true;
+
+    if (!cache) {
+        cache = flightgear::NavDataCache::createInstance();
+        doingRebuild = cache->isRebuildRequired();
+    }
+
+    if (doingRebuild) {
         bool finished = cache->rebuild();
         if (!finished) {
             // sleep to give the rebuild thread more time

src/Navaids/NavDataCache.cxx

@@ -1129,12 +1129,14 @@ NavDataCache::~NavDataCache()
   d.reset();
 }
 
-NavDataCache* NavDataCache::instance()
+NavDataCache* NavDataCache::createInstance()
 {
-  if (!static_instance) {
     static_instance = new NavDataCache;
+    return static_instance;
 }
     
+NavDataCache* NavDataCache::instance()
+{
   return static_instance;
 }
   

src/Navaids/NavDataCache.hxx

@@ -58,6 +58,9 @@ public:
 // singleton accessor
     static NavDataCache* instance();
 
+// static creator
+    static NavDataCache* createInstance();
+
     SGPath path() const;
     
   /**