window: Derive from GtkWidget

window: Derive from GtkWidget

We want to remove GtkBin and GtkContainer as they don't
provide much useful functionality anymore. This requires
us to move get_request_mode and compute_expand down.

Update the accessible implementation to match, remove
remnants of container implementations in GtkWindow
subclasses, and fix livecycle issues around destroy
vs dispose in GtkAssistant.

After this commit, using gtk_container_add on window
subclasses is not allowed anymore, but adding childing
with <child> in ui files still works.

See #2681