xfce.bbclass: Add further folders for ${PN}-dev/-staticdev/-dbg

* Some panel-plugins (e.g xfce4-clipman-plugin) store libraries in
  /usr/lib/xfce4/panel/plugins instead of
  /usr/lib/xfce4/panel-plugins.
* build tested on maiden tmp-dir
* run tested all applications and plugins on overo

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