dbus: add startShmSink

Since the video split, the daemon can manages a lot of Sink clients
which are generally not used by any client. This introduces a lot
of wasted computation.
One of the problems is that every sink client have a SHM memory if
dbus is enabled and any update generates frames for this SHM memory
even if the client is not showing them.
This patch introduces VideoManager::startShmSink, which enable
or disable transfer and works like VideoManager::registerSinkTarget.
Thus, a transfer will only be done if the client is explicitly
asking to transfer frames.

Change-Id: I1d265b7ffcdc37aff9c5f729a146fa26c5d7d4a1
GitLab: https://git.jami.net/savoirfairelinux/jami-product-backlog/-/issues/9