Dispatch dbus messages after method calls when no main loop running

If no main loop is running, then the dbus connection's incoming message
queue was never emptied, so with, eg, dogtail,
dbus_connection_send_with_reply_and_block would take longer and longer
to look through the message queue to find a reply. This also effectively
led to a memory leak.

https://bugzilla.gnome.org/show_bug.cgi?id=701145