DolphinQt: Properly handle quit events.

* Confirm stopping emulation when the window is closing, not just the "Stop" button
 * Don't resume if we were already paused when we got the quit event
 * Shutdown the core at the end of main() so we don't crash on exit
 * Miscellaneous other logic cleanups related to this

Source/Core/DolphinQt/MainWindow.h

@@ -44,7 +44,6 @@ private slots:
 
 	// Main toolbar
 	void OnBrowse();
-	void OnExit();
 	void OnPlay();
 	void OnReset();