a11y: Unrealize ATContext on dispose()

a11y: Unrealize ATContext on dispose()

By unrealizing the context we avoid additional work during the dispose
phase, in case widget code updates the accessible state. We use
GtkAccessible's API, to ensure we unrealize the right ATContext, instead
of the one we store inside GtkWidgetPrivate.

We drop the ATContext instance inside GtkWidget during finalization, to
mop up eventual vivifications there.