gdbus: Fix sending ObjectManager/Properties signals out of order

gdbus: Fix sending ObjectManager/Properties signals out of order

In some cases the order of the messages is altered when a message is
sent without processing the pending signals first, currently this affect
client_check_order unit test:

/gdbus/client_check_order: **
ERROR:unit/test-gdbus-client.c:795:property_check_order: assertion failed: (g_strcmp0(string, "value1") == 0)

As can be observed the value of the property is not yet updated because the
signal it is still pending, once this fix is applied the test pass:

/gdbus/client_check_order: OK

Note that the flushing only works when g_dbus_send_message is used so
places where dbus_connection_send and other variants are called directly
may still change the order.