Add warning popup when loading save states for the first time (#6565)

* citra_qt: Remove global state usage in GMainWindow

* citra_qt: Add warning when loadings saves for the first time

* citra_qt: Focus window when launching game from cmdline

* citra_qt: Cleanup nullptr checks

* citra_qt: Move setting to UISettings

* renderer_opengl: Remove header

src/citra_qt/bootmanager.cpp

@@ -78,7 +78,6 @@ void EmuThread::run() {
         });
 
     emit LoadProgress(VideoCore::LoadCallbackStage::Complete, 0, 0);
-    emit HideLoadingScreen();
 
     core_context.MakeCurrent();