Shuffle the code around and make use of docstrings to conform to
the newly introduced coding style as described in `HACKING.md`
This commit also introduces docstrings describing each source file.
introduce `calls_sip_media_manager_get_capabilities ()` which takes
a GList of MediaCodecInfo's as input to generate a SDP message.
If using in an SDP answer we simply feed it a list of the common codecs
as gathered from the SDP offer.
static capabilities are now supporting multiple codecs and generate correct SDP
messages. This only relates to SDP offers. When we are answering an offer we
must take the offer from the other party into account when creating the answer.
Hence why we differentiate between "static" and "dynamic" (not implemented yet)
capability strings.
