LibGUI: GDialog should close its nested event loop on window close.

Make GWindow::close() so we can override it in GDialog and quit from the
internal event loop when the window manager tells us to close ourselves.

The dialog will return GDialog::ExecCancel in these situations.

Libraries/LibGUI/GDialog.cpp

@@ -40,3 +40,10 @@ void GDialog::done(int result)
     dbgprintf("%s: quit event loop with result %d\n", class_name(), result);
     m_event_loop->quit(result);
 }
+
+void GDialog::close()
+{
+    GWindow::close();
+    m_event_loop->quit(ExecCancel);
+}
+