Create a 'mirror' model of the proxy for the testcase. In addition to
testing that the proxy model emits the proper signals this also keeps
the proxy alive (by holding references to it from the mirror).
The previous code would create the submenu proxies and destroy them
right away (from the recursive step in the equality comparison
functions). This means that the subscription would go out over D-Bus
and the proxy would be destroyed before it returned. Keeping the model
alive allows it to be actually updated.
Each test needs to remove the sources that it attaches
to the default main context, or else things will work
fine in isolation, but go bad in a full test run.
There are no public 'exporter' objects, so don't allude to them
in the function names. At the same time, we want to make it clear
that these functions are D-Bus specific.
The new APIs are
g_action_group_dbus_export_start
g_action_group_dbus_export_query
g_action_group_dbus_export_stop
g_menu_model_dbus_export_start
g_menu_model_dbus_export_query
g_menu_model_dbus_export_stop
