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/main.cpp

@@ -408,16 +408,8 @@ int main( int argc, char* argv[] )
 
         // The OverlayController handles the majority of the application specfic
         // things. It contains all the other tabs.
-        // It is created using a singleton pattern that destructs the old
-        // instance whenever createInstance is called. This is obviously not
-        // ideal since it can lead to dangling points and use after free.
-        // The constructor only sets two member vars called desktopMode and
-        // noSound, the rest of the initialization is done in Init. It is
-        // unknown why this is done.
-        advsettings::OverlayController* controller
-            = advsettings::OverlayController::createInstance( desktopMode,
-                                                              noSound );
-        controller->Init( &qmlEngine );
+        advsettings::OverlayController controller(
+            desktopMode, noSound, qmlEngine );
 
         QQmlComponent component(
             &qmlEngine, QUrl::fromLocalFile( "res/qml/mainwidget.qml" ) );
@@ -428,7 +420,7 @@ int main( int argc, char* argv[] )
                          << std::endl;
         }
         auto quickObj = component.create();
-        controller->SetWidget(
+        controller.SetWidget(
             qobject_cast<QQuickItem*>( quickObj ),
             advsettings::OverlayController::applicationDisplayName,
             advsettings::OverlayController::applicationKey );