WifiDisplay uses RTSP for stream-setups. This adds a basic rtsp-bus
implementations that we can use for sinks and sources.
Note that the implementation is optimized for usability, not speed. RTSP
is used for control-data, not streaming-data so there's no need to
over-optimize it. In case inlined RTP is used, we still provide proper
speed, even though that's usually not used by WifiDisplay implementations
(due to the TCP requirement).
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
