GActionGroupExporter: stop using signal IDs

GActionGroupExporter: stop using signal IDs

GDBusConnection recently changed to dispatching its GDestroyNotify calls
from an idle instead of on-the-spot.  Under the previous regime, we
would destroy-notify the action group export of a GtkApplicationWindow
at the point it was removed from the application (ie: slightly before
being disposed).

With the destroy notify now deferred to an idle, the window has already
been disposed, so the signal handlers have already been disconnected.

Avoid the problem by dropping our use of signal IDs and just do
g_signal_handlers_disconnect_by_func(), which doesn't complain if there
is no connection.