telepathyClient: ignore invalidated channels

telepathyClient: ignore invalidated channels

There is a race if a channel is invalidated during its preparation: the
'invalidated' signal is already emitted so the Shell will never notice.
We fix this by simply checking if the channel is already invalidated when
receiving it from telepathy-glib.

In the approving case, we reject the full ChannelDispatchOperation as we only
support approving one channel at the time.

https://bugzilla.gnome.org/show_bug.cgi?id=677457