This abstracts the machinery to update the opaque region for a GtkWindow
into GtkNative so it may be used from other native impelementations such
as GtkPopover.
Related #4689
This changes allocation of the widget trees to happen as a side effect
to the GdkSurface::layout signal, which first passes the GtkNative
instance where it is then forwarded to the implementations of the
GtkNative interface.
The implementations of GtkNative are the ones doing the actual
gtk_widget_allocate(), and they do so in their GtkNativeClass::layout
function.
