Run GTK main loop in between incoming stanzas
- This prevents 30-60 second lockups when starting gajim while the main thread is processing large numbers of incoming stanzas and not yielding to the GTK main loop at all.
- This is done by explicitly running the main loop after a stanza is received, if there are events pending.
(Empirically, this makes gajim actually usable on my machine, whereas current master is pretty bad lockup-wise after startup/reconnection...)