This is the first step in adding support for multiple
streams per endpoint. Each source(stream?) needs to have
an identifier. For now always generate the 0 index name
until the machinery for sending more than 1 stream is put
in place.

* send only MSID attribute

* add feature flag for source name signaling

* log a msg if source name signaling is enabled

SDP utility classes are spread across RTC and XMPP directories now, moving these class files to a 'sdp' directory.

using more specific selectors

to test the mapping of fmtp lines which are not key=value pairs

follow-up from #1146. Very high level but this should cover the relevant code paths.

The jingle offer was taken from a current focus offer with a few bits removed:
* ssrc-info element
* web-socket element inside transport
* rtcp-mux element inside transport
* bridge-session element
* bundle element (removed in focus already)

Ssrcs not grouped together in the jingle element
are not recognized by the bridge. When parsing an
sdp with ssrcs that are not listed consecutively
it is important to still group them together
in the jingle element.