Change OverlayController from singleton pattern to regular RAII

The singleton pattern was poorly implemented and didn't provide the
benefits usually associated with the singleton pattern, with all the
negatives.

Refer to this github issue:
https://github.com/OpenVR-Advanced-Settings/OpenVR-AdvancedSettings/issues/29

src/overlaycontroller.h

@@ -94,16 +94,12 @@ public: // I know it's an ugly hack to make them public to enable external
     AccessibilityTabController accessibilityTabController;
 
 private:
-    OverlayController( bool desktopMode, bool noSound )
-        : QObject(), desktopMode( desktopMode ), noSound( noSound )
-    {
-    }
     QPoint getMousePositionForEvent( vr::VREvent_Mouse_t mouse );
 
 public:
+    OverlayController( bool desktopMode, bool noSound, QQmlEngine& qmlEngine );
     virtual ~OverlayController();
 
-    void Init( QQmlEngine* qmlEngine );
     void Shutdown();
 
     bool isDashboardVisible()
@@ -174,21 +170,8 @@ signals:
 
 private:
     static QSettings* _appSettings;
-    static std::unique_ptr<OverlayController> singleton;
 
 public:
-    static OverlayController* getInstance()
-    {
-        return singleton.get();
-    }
-
-    static OverlayController* createInstance( bool desktopMode, bool noSound )
-    {
-        singleton.reset(
-            new advsettings::OverlayController( desktopMode, noSound ) );
-        return singleton.get();
-    }
-
     static QSettings* appSettings()
     {
         return _appSettings;