xfce4-orage: reflect changed dependencies after Xfce 4.16 update

Xfce 4.16 no longer supports GTK2 panel plugins, so while this package
still builds and runs otherwise, it no longer builds a panel component.
(It also has no GTK3 dependency, so stop pulling in such from the xfce4
meta-pkg, of which it is no longer a part, since upstream considers it
EOL.)

(gutteridge)