Add GtkATContext

Add GtkATContext

The ATContext type is meant to be used as the base class for
implementations of the assistive technology API—the actual mechanism
needed to communicate to components like the screen reader, or any other
AT.

Every time the widget state changes, the ATContext is meant to broadcast
the state change; and every time the AT queries the state of a UI
element, the ATContext is meant to provide that information.

We also have a "test" ATContext implementation, which is meant to be
used to write tests to verify that changes are propagated without
requiring a whole desktop session.