fs-element-added-notifier: do not unref elements from already added bins when setting a keyfile

The FsElementAddedNotifier does not own a reference to these elements.
rtpsession: Only try to read RTCP if mapping succeeded
example: Sources need to be live

In current state this commandline example produce choppy audio,
with warning: "Unexpected discontinuity in audio timestamps of
-0:00:03.168625000, resyncing". Setting the source to live,
as it's suppose to be, fixes the issue.
rtpcodeccache: Add input/output caps to the codec cache
rtpcodecdisco: Discover input caps from send codecbin
rtpcodecdisco: Discover output caps from receive codecbin
rtp: Use FsStreamDirection instead of "is_send" gboolean

Makes the API clearer
rtpcodecnego: Add CodecPreference struct wrapping codec prefs
rtpdiscocodec: Factor out in/out caps discovery
rtpcodecnego: Discover input/output caps for application specified pipelines

From the codec preferences, if there is a pipeline, inspect to find the
possible caps.
rtpcodecnego: Skip full list iteration

One send codec is enough to be happy!
rtpsession: Add input and output caps
session: Add API to set the allowed caps
rtpsession: Implement allowed caps setting
rtpcodecnego: Filter by input and output caps
rtpbitrateadapter: Remove gray caps
rtpbitrateadapter: Pass media type from caller
rtpbitrateadapter: Remove caps property
rtpbitrateadapter: Let upstream do possible renegotiation on > 10% bitrate change
rtpbitrateadapter: Do dynamic getcaps