telepathyClient: send typing notifications

telepathyClient: send typing notifications

If we're typing we want to send composing. If we empty the entry we
want to send active. If we're typing but don't type any more for
COMPOSING_STOP_TIMEOUT seconds, we want to send paused. Simple.

This behaviour was stolen from Empathy where it has won many awards.

https://bugzilla.gnome.org/show_bug.cgi?id=650196

Based on patch from Jonny Lamb <jonnylamb@gnome.org>
Signed-off-by: Alban Crequy <alban.crequy@collabora.co.uk>