Tightened up the code for call channel group membership changes
Tightened up the code for call channel group membership changes

The actual changes and emissions are now concentrated in the callback
for protocol session state changes, plus the cases when a call termination
reason and text is obtained from the protocol messages.

freedesktop-unofficial-mirror/telepathy__telepathy-rakia,freedesktop-unofficial-mirror/telepathy__telepathy-rakia,freedesktop-unofficial-mirror/telepathy__telepathy-rakia