Add a simple close button ("X") to windows.

Clicking the button generates a WindowCloseRequest event which the client app
then has to deal with. The default behavior for GWindow is to close() itself.

I also added a flag, GWindow::should_exit_event_loop_on_close() which does
what it sounds like it does.

This patch exposed some bugs in GWindow and GWidget teardown.

LibGUI/GEvent.h

@@ -24,6 +24,7 @@ public:
         WindowBecameActive,
         FocusIn,
         FocusOut,
+        WindowCloseRequest,
     };
 
     GEvent() { }