[mythtv-cmyth] Add demuxer

This adds the demuxer implementation.
The work is partially based on the VNSI demuxer.
[mythtv-cmyth] Use demuxer in addon

Build the demuxer and use it in the addon.
New setting 'demuxing' in category 'advanced' allows to enable/disable the
functionality.
[mythtv-cmyth] GetPIDStream: No need to find in the map