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
window-actor: Move signal handler connections to the meta-window setter