Allow other clients to preempt the channels we are handling

Allow other clients to preempt the channels we are handling

This is needed if we are handling an incoming text channel and then user tries
to open a chat with the same contact using Empathy. In this case, the Shell
should delegate the channel back to Empathy and just continue observing it as
it does for usual outgoing channels.

Depends on telepathy-glib 0.15.3 as
tp_base_client_set_delegated_channels_callback() has been added in this
version.

https://bugzilla.gnome.org/show_bug.cgi?id=654237