transport: Implement retransmission

This enable retransmission in OwrTransportAgent. We look up
retransmission paramters, if they exist, for the payloads of a media
session, and if they do, we add retransmission elements to rtpbin and
enable retransmission on the corresponding RTP streams.
transport: Drop unused OwrTransportAgentPrivate member
transport: Be a bit smarter about setting up rtpbin for sending

This makes us not indiscriminately set up the rtpbin as a sender, but
only when we require it. We do need to make sure that we set up RTCP
sending even if we're not sending RTP, though.

Other than just cleaning up, this is also needed because rtpbin does not
send out RTCP packets when enabled as a sender, until the first RTP
packet is sent. In the case where we only receive media, this means RTCP
packets would never be sent out.
transport: Add some code to test retransmission

This is off by default, but if we compiled in, there is an identity
element before we feed RT(C)P packets to rtpbin which drops some
packets, emulating packet loss.
tests: Enable retransmission for video in test-send-receive

This is on by default here, but needs TEST_RTX to be defined in
transport/owr_transport_agent.c to generate some actual data loss and
trigger retransmission.
transport: Add gtk-doc annotations to new internal API
transport: Allow looking up OwrPayload by rtx payload type

This is currently useful in the rtpbin request-pt-map signal handler,
where we might need to look up a payload by pt or rtx-pt.