FIX: exit from tray

If you specify "minimize_on_close" in the settings, there is no way to exit the application, currently. The tray icon just vanishes, but the app keeps running, without any GUI.

As you can launch the app multiple times, you end up with multiple unreachable instances running.

This quit the application if "Exit" is selected from the tray icon, which is what users would expect anyway, imho.