media-manager: Don't put InitialVideo in the allowed caps, ring can't do video
media-manager: sanatize requestable channels

Call channels can only be requested via Create & Ensure and don't
support the compatibility mode of starting with an unknown handle, so
remove support for that.

Also given the contents aren't mutable on a gsm call, make
InitialAudio=True a fixed property.
call-channel: Remove the CallState interface

The CallState interface is not meant for Call changes, all the
information needed is in the remote call member flags and the call
flags. As a first step remote CallState as it breaks some signals from
the Call interface and add TODO items for reimplementing things
correctly using the new spec work.
call: Remove BaseCallChannel and CallMember

The base-call channel and call-member infrastructure was copied for
Gabble where it mostly is meant for Muji calls (muc video calls) where
each member in a call needs to have its own jingle session. In Ring one
call channel matches one remote target and one modem session, so all
this infrastructure is unneeded and in the current state didn't really
do much...

Also remove the handle of add-content, ring channels only have one audio
content from the start, always. And remove compatibility code for
starting calls to an unknown handle.
call-channel: Remove group interface usage

The current group interface usage was a left-over from the StreamedMedia
days were all call control was done using a group. Remove this and
properly implement accept for outgoing calls (trigger the start of the
dail) and hangup (actually hang up the call even if the channel isn't
open).

The group interface may come back later if we (re-)implement
conferencing using Call channels.
Remove last tp_group calls