sip_session: change UI message

darcs-hash:20090326084636-b2873-115dbcebe10d0257a15e2369b4724cf11c4f57e4
sip_session: add :hold, :unhold, :add chat|audio, :remove chat|audio commands

darcs-hash:20090326084706-b2873-91fc04478be827b6420ddaeac687d503e135fdfa