Implement a Telepathy Approver and Handler (#643594)

As the Shell does more than observing channels (users can interact with them),
it has to be an Handler as well. We have to make sure that all the new
incoming text channels are handled by the Shell by default, so we make it an
Approver as well.

From an user point of view, the only difference is that Empathy's tray icon
will stop blicking when receiving new channels.

We rely on ChannelDispatcher.DelegateChannels() and PresentChannel() to
interact with Empathy. Those methods have been implemented in
telepathy-mission-control 5.9.0 and telepathy-glib 0.15.0.