telepathyClient: Disentangle source and notification

telepathyClient: Disentangle source and notification

Currently the lifetime of a chat source and its single notification
are tied together. While this apparently makes sense, it means we
will lose all follow-up notifications when a source is destroyed
with the corresponding telepathy channel left open. We will fix this
soon by tying the source to the channel's lifetime rather than the
notification, prepare for this by recreating the notification if
necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=747636