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... 
It's good enough, I think.
The goal is to check if there is a X server. If not and if you didn't start SABnzbd with -d, then pygtk will complain (emit some error message without making sabnzbd stop or crash). That's what I wanted to avoid.
But yes, if DISPLAY is set incorrectly (does not point to a X server), then my check is useless: pygtk will complain.