conference: allow correct dominant speaker detection
Canonical identifier in RTCP packets sent during media call should be set
to Globally Routable UA URI (GRUU) of the Sipe instance.
MSOC clients use this value to detect speaking persons in voice conference
and in the list of participants display a different "talking telephone receiver"
icon next to the one who is speaking right now.
Libpurple patch is required to allow setting of SDES parameters of underlying
Farsight conference, reported upstream as http://developer.pidgin.im/ticket/12981