Linux: Replace GUI with notification bubbles

[ghost]

I was getting a little annoyed by the GUI window. It pops up every time that a game has finished idling, and steals my input focus, which can be frustrating. This PR replaces the window with a notification bubble, which makes more sense anyway, because there is nothing interactive about that window.

[ghost]

Notable change: steam-idle.py will now exit after showing the notification, rather than entering a GUI main loop. Card drops are still working for me just fine with this, though.

[frostworx]

great feature. works excellent. thank you very much, scrawl (also for your hard work on openmw :})!

[barnaba]

I'd like to see this merged.