xfce4-14: move to xfce and have only one version of xfce

All code that was at xfce4-14 has been moved to xfce/*.
Old expressions that aren't rewritten might be abandoned or broken.

Additonally I've ported the xfce4-14 thunar expression to support
thunarPlugins. We can now support this interface in the Xfce module
again, although I'm not sure if we have any plugins packaged that support
latest thunar.