Work around the GNOME Shell issues related to switcher presentation

Orca used to automatically present the selected item in a newly-shown
switcher. It doesn't in GNOME 3.14. This is due to a couple of issues:

* The ordering of accessible events has changed, so first we get the
  accessible event for the selected item. Then we get the one for the
  window activation. As a result, Orca ignores the selected item because
  it doesn't yet know the user is in a GNOME Shell window. (bgo738701)

* The subsequent focused event for the containing panel, which does come
  after the window activation, should have be enough to cause Orca to
  automatically present the selected item even with the event ordering
  bug above. But this fails because Orca expects containers which contain
  selectable children to implement the accessible selection interface.
  GNOME Shell's switchers don't. (bgo738705)

This commit hacks around the second issue and is sufficient to cause
Orca to present the initially-selected item in a newly-shown switcher.