Linux systray icon

This code adds a systray icon for Linux systems, inspired from the Windows one.
This works on all desktops (Gnome, KDE, XFCE) except Ubuntu Unity (would require using appindicator).
It can be disabled using the same option as the Windows systray icon (win_menu).
It fails gracefully if pygtk is not available so GTK/X are hopefully not new requirements to run sabnzbd...