gnome-shell 3.12 basic support

In gnome-shell 3.12 the signals have been moved from MetaWindowActor to
MetaWindow.

With this patch shellshape is starting in gnome-shell 3.12 but still seems to
be unstable. Maybe the 3.12 development can be done in a working branch on
master side, so more people are able to test and fix bugs?

This is pull-request is related to #132