Port to xfconf

Please note that I took the quick path of just exchanging the backend (vs. the actually nice fix of using GObject properties and bindings, but that would have meant much more refactoring and work)...

So you cannot change properties in xfconf and expect the plugin to be updated. There are several other benefits though, e.g. the plugin's config being backed up by xfce4-panel-profiles properly/easily etc.