xfce-panel-plugin: bbclass for panel plugins

* based on Koen's patch [1]
* use libxfce4ui instead of libxfcegui4

[1] http://lists.linuxtogo.org/pipermail/openembedded-devel/2011-October/035589.html



Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>