LibGUI: Exit the main event loop when the last window is deleted.

This behavior is the new opt-out default. If you don't want your app to exit
when the last GWindow is destroyed, call this:

    - void GApplication::set_quit_set_quit_when_last_window_deleted(bool)

Also renamed "windows()" to "reified_windows" in GWindow.cpp to reflect that
it only contains GWindows that have a server-side representation. :^)

Libraries/LibGUI/GApplication.cpp

@@ -111,3 +111,9 @@ void GApplication::hide_tooltip()
     if (m_tooltip_window)
         m_tooltip_window->hide();
 }
+
+void GApplication::did_delete_last_window(Badge<GWindow>)
+{
+    if (m_quit_when_last_window_deleted)
+        m_event_loop->quit(0);
+}