For now only WebRTC messages can be listened to, although it will be
extended with other kinds later.
This commit only introduces the base class, although it is not used yet
anywhere; a concrete implementation will be added in a following commit.
The test class is named "SignalingMessageReceiverWebRtcTest" rather than
just "SignalingMessageReceiverTest" to have smaller, more manageable
test classes for each listener kind rather than one large test class for
all of them.
Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>
