a11y: Rework ownership and lifetime of GtkATContext

a11y: Rework ownership and lifetime of GtkATContext

Now that GtkATContext is explicitly realized and unrealized, we should
always create an instance at widget initialization time, and drop it
during the widget finalization. This should make it easier to set up the
initial accessible state of a widget during the instance initialization,
as well as reduce the chances of accidental creation of GtkATContext
instances during the destruction sequence.