Removing parts from xfce4gui4

[Nick Schermer]

I'd like to drop some parts from libxfcegui4 (trunk):

- xfce_clock.{c,h}
- xfce_systemtray.{c,h}
- Some marshals used by the tray code in xfce_marshal.list

Both have been moved into the panel, so they are not needed anymore.
If nobody rejects, I'll remove the pieces in a couple of days.

We could also drop xfce_marshal.{c,h} since they are autogenerated.

Greets,
Nick