Although "unshareScreen" is technically bound to a specific peer
connection it is instead treated as a general message on the call
participant.
Nevertheless, call participant messages will make possible (at a later
point) to listen to events like "raise hand" or "mute" (which, again,
could be technically bound to a specific peer connection, but at least
for now are treated as a general message on the call participant).
Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>
