compositor: Delay meta_compositor_add_window until the first show

In order for the compositor to properly determine whether a client
is an X11 client or not, we need to wait until XWayland calls
set_window_id to mark the surface as an XWayland client. To prevent
the compositor from getting tripped up over this, make sure that
the window has been fully initialized by the time we call
meta_compositor_add_window.

https://bugzilla.gnome.org/show_bug.cgi?id=720631
Move position-changed / size-changed signals to the MetaWindow

They fit more appropriately over here...

https://bugzilla.gnome.org/show_bug.cgi?id=720631