libxfce4ui: build gtk3 libs and pack gtk2/gtk3 libs in separate packages

the solution provided still allows gtk3 free images (tested on image including
packagegroup-xfce-extended) but there will be updates/new recipes for xfce4
requiring gtk3 part of libxfce4ui e.g xfce4-pulseaudio-plugin (replacement for
deprecated xfce4-mixer due to missing mixer API in gstreamer-1.0) / parole /
xfce4-power-manager.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>