New system tray in xfce4-panel

[Nick Schermer]

I've rewritten most of the tray code, including the manager (which can
be removed from libxfcegui4). Hopefully this will fix most of the
problems related to the icon size. New features include multiple
lines, hidden applications and sorted by application name.

Tray icon messages are not fully integrated yet (but it never was).
I'll commit this soon. Also transparent icons are not working yet, but
I first need a good way to fix this.

In the mean time the code can be tested so I can polish it a bit here and there.

Have fun,
Nick

Screenshot: http://foo-projects.org/~nick/tmp/tray-snap.png