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/VideoInterface/RenderWidget.h

@@ -19,10 +19,7 @@ protected:
 	void mousePressEvent(QMouseEvent*) override {}
 	void paintEvent(QPaintEvent*) override {}
 
-private slots:
+private:
 	void closeEvent(QCloseEvent* e) override;
-
-signals:
-	void Closed();
 };