xfce4-dev-tools: cleanups

* do not inherit xfce class to avoid unwanted native dependencies
* xfce4-dev-tools.ipk contains complete set of xfce developer extensions
* run tested on fresh build dir / image similar to angstrom xfce-nm-image
  (task-xfce-extended included) / target overo

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