New clock plugin in xfce4-panel trunk

[Nick Schermer]

I've rewritten the clock plugin in the panel. It now uses cairo for
most of it's drawing and features a new binary clock (both true binary
and bcd). Further more, you can now now set custom time and tooltip
formats to suite your needs.

This also means we can remove the clock code from libxfcegui4 - trunk.
If nobody has something agains this, I'll remove it in a couple of
days.

Xfce4-panel now also depends on cairo and gtk 2.8.0 for cairo usage,
so Jasper, maybe fix the cairo transparency patch and commit it?

So feel free to try it and drop your comments and stuff on the ml,
I'll fix them ASAP.

Cheers,
Nick