x11-wm/compiz[gnome] stuff seems unnecessary and only serves to depend on deprecated library slots. At least the gnome-control-center and gnome-desktop parts - the metacity part might be useful for some cases, if the theming format is similar enough, but for gnome3 not so much (client-side decorations with header bars don't really do this kind of theme..). So USE=gnome for this is questionable these days. Please consider getting rid of gnome-desktop:2 and gnome-control-center:2 dependencies. The gnome-control-center one seems to be about gnome-window-settings or something, while no-one really stores these settings in there anymore, as I believe things like MATE have their own locations for these kind of settings. gconf isn't really used either. I can't really find what gnome-desktop is linked for, maybe gnome_window_manager_get_type was from there, not sure. It's time to clean things up regarding ancient GNOME2 library slots, please stop using them, thanks.
this was removed
