Desktop effects activation

After having implemented the same functionality already available in Fedora and Ubuntu (different implementations of the Desktop Effects tab on the appearance control-center applet) for openSUSE, it is time to come to a common solution for all distros to remove their need to add this functionality. An initial patch is available here.

Still lots of things need to be discussed and clarified, so if you care about activating desktop effects in GNOME, GNOME Control Center mailing list.