Exporting can only be done relative to a particular given main context
and all interaction with the action group must be on that same context.
Fix up the implementation so that the user can specify that context with
the normal (thread default) mechanism and document the limitation on the
API.
Adjust the testcase to adhere to the documentation limitations. It
passes now.
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