Fix GtkApplicationWindow action group implementation

GtkApplicationWindow frees its internal action group on dispose for the
usual reasons: to avoid the possibility of reference cycles caused by
actions referring back to the window again.

Unfortunately, if it happens to be inside of a GtkActionMuxer at the
time that it is disposed, it will (eventually) be removed from the muxer
after it has been disposed.  Removing an action group from a muxer
involves a call to g_action_group_list_actions() which will crash
because the internal action group to which we normally delegate the call
has been freed.

A future patch that reworks the quartz menu code will introduce a use of
GtkActionMuxer in a way that causes exactly this problem.

We can guard against the problem in a number of ways.

First, we can avoid the entire situation by ensuring that we are removed
from the muxer before we destroy the action group.  To this end, we
delay destruction of the action group until after the chain-up to the
dispose of GtkWindow (which is where the window is removed from the
GtkApplication).

Secondly, we can add checks to each of our GActionGroup and GActionMap
implementation functions to check that the internal action group is
still alive before we attempt to delegate to it.

We have to be careful, though: because our _list_actions() call will
suddenly be returning an empty list, people watching the group from
outside will have expected to see "action-removed" calls for the
now-missing items.  Make sure we send those. but only if someone is
watching.

https://bugzilla.gnome.org/show_bug.cgi?id=710351

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