Another take at deprecation warnings.

This also fixes the system tray menu visually, since the KDE4 one has its own minimize/restore action and thus currently showed two "Restore" entries.

The change also hides the quit action which is now also standard in system menus.

It would probably be better to disconnect from the maybeQuit slots and connect it to the main window's quit handling code instead, which warns that PPP will also be killed and shuts down the network properly.