st: implement ClutterActorClass.has_accessible()

Implement ClutterActorClass.has_accessible() to ensure that CallyActor does not recreate accessibles during the removal/destruction of an actor. This relies on mutter!1083 (merged) for the ClutterActorClass.has_accessible virtual function.

Running GNOME Shell for about 30 seconds results in a difference between the two runs.

Before:

    ALLOCATED      TOTAL    FUNCTION
[   52.2 KiB] [   0.05%]    cally_actor_real_remove_actor
[   36.3 KiB] [   0.04%]      st_widget_get_accessible
[    9.8 KiB] [   0.01%]      atk_gobject_accessible_for_object
[    3.2 KiB] [   0.00%]      g_signal_emit_by_name
[    2.9 KiB] [   0.00%]      clutter_actor_get_children

After:

    ALLOCATED      TOTAL    FUNCTION
[    1.8 KiB] [   0.00%]    cally_actor_real_remove_actor
[    1.1 KiB] [   0.00%]      clutter_actor_get_children
[  659 bytes] [   0.00%]      g_signal_emit_by_name

Obviously 50KiB isn't a huge savings.

Although fixing things to avoid re-entrancy on destruction can be very useful from a correctness standpoint.

Fixes #2263 (closed)