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
compositor: Remove atom_x_root_pixmap

It's unused
Move _NET_WM_WINDOW_OPACITY handling to the standard window-props interface
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
window-actor: Remove suspicious optimization

Calling get_paint_volume() should not modify internal state.
surface-actor: Move unobscured_region processing here
window-actor: Remove outdated code path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-actor: Never unredirect when under Wayland
window-actor: Simplify the unredirected check in cull_out
compositor: Simplify the unredirected window management code
window-actor: Flip set_redirected around

I know it's confusing with the triple negative, but unredirected is how
we track it elsewhere: we have an 'unredirected' flag, and 'should_unredirect'.
window-actor: Remove old unused APIs
Move position-changed / size-changed signals to the MetaWindow

They fit more appropriately over here...
surface content
xwayland: Update the surface actor for the window on set_window_id

We need to do this for XWayland windows, since we only get the event
telling us they're an XWayland window after the compositor knows about
the window.
window-actor: Don't use TFP under XWayland

Simply have a NULL surface actor until the set_window_id arrives...