LibGUI: Added window creation callback to GApplication/GWindow

Added a window creation callback to GApplication that gets called
by GWindow which will reset any pending exit request in the
CEventLoop.

This is to prevent a bug which prevents your application from
starting up if you had a message box or other dialog before
showing your main application form. The bug was triggered by
there being no more visible windows which was triggering a
premature quit().

Libraries/LibGUI/GWindow.cpp

@@ -84,6 +84,7 @@ void GWindow::show()
     apply_icon();
 
     reified_windows.set(m_window_id, this);
+    GApplication::the().did_create_window({});
     update();
 }