call: fix OCP violation

"RoboCop watching you"

IM dedicated code has been inserted into Call::setState()
and it's an Open Close Principle violation.
Move it into a dedicated function and assign it to call state
change as listener.

As code changed, also make a better state listening implementation
that do not depend on listener call order.

Notice: TODO have been inserted, it's for another patch.

Change-Id: Ibe37555320de1dec173338a62323320d3d93bb4a
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>