Refactor how shapes are done

As part of Wayland support, we should hold the shape and opaque regions
on the MetaWindow rather than fetching them in the MetaWindowActor, as
this gives us better flexibility as to where the regions are set, and
allows for easier Wayland support.

To make merging easier with the Wayland branch, we also append the _x11
suffix to functions that use the X SHAPE extension to fetch the shaped
regions.

https://bugzilla.gnome.org/show_bug.cgi?id=706930