window-actor: Remove outdated code path

The shadow is added in the paint step, not as a separate actor,
so the raise is a no-op. It also gets rid of an annoying misspelling
that's driving me crazy.
window: Add a simple meta_window_get_toplevel_xwindow utility

To replace all the places where we do:

  window->frame ? window->frame->xwindow : window->xwindow

or similar...
window-actor: Remove (private) xwindow property

This should make it easier to construct a MetaWindowActor from bindings.

https://bugzilla.gnome.org/show_bug.cgi?id=678989
window-actor: Remove meta-screen property

This can be inferred from the window. Don't allow anybody to set it.

https://bugzilla.gnome.org/show_bug.cgi?id=678989
window-actor: Make the meta-window property construct-only

We don't deal with setting this property at all, and it's unlikely
to come up in future settings, so just don't allow setting it at all.

https://bugzilla.gnome.org/show_bug.cgi?id=678989
Allow potentially supporting multiple MetaWindowActors per window

A lot of the code in mutter assumed a one to one relationship between a
MetaWindow and a MetaWindowActor, and that it owns its
MetaWindowActor. For the most part, we've gotten around this in GNOME
Shell by creating a ClutterClone of the MetaWindowActor. In some spots, we
reparent the actor owned by Mutter, and use it for our own purpose, which
just plain isn't nice. Since the Clutter maintainers have plans to kill
off ClutterClone, and since MetaWindowActor is already a very thin wrapper
around MetaShapedTexture, we should be able to create multiple
MetaWindowActors for a MetaShapedTexture.

This commit replaces the "compositor_private" property on MetaWindow with
a direct reference to a MetaWindowActor (which may go away at some point,
see below), and a list of window actors.

The direct reference is a replacement for the "compositor_private"
property that has been there previously. It doesn't make sense to run
mutter without compositing, so there's no real reason to keep the veil of
a compositor/core split.

The eventual plan I have in mind is to remove the direct reference to
MetaWindowActor, and have the compositor/plugin just create a new window
actor like anybody else.

https://bugzilla.gnome.org/show_bug.cgi?id=678989
window-actor: Don't do any special initialization in _new

https://bugzilla.gnome.org/show_bug.cgi?id=678989