gtk-xfce-engine: create and pack gtk3 themes by default

we already have some gtk3 apps in a typical image: xfce4-pulseaudio-plugin,
network-manager-applet and xfce4-power-manager 1.5.x will come soon to
meta-xfce. These are using gtk3 themes (which need love - but this is a
different story).

Without this patch loading Xfce gtk3 themes causes complaints for missing
xfce theme engine.

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