[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] Implement timeshift functions (PVR API 1.9.0)

- GetPlayingTime()
  Available only when using the internal MPEG-TS demuxer. Otherwise, return
  default value to PVR.

- GetBufferStartTime()
  Returns start time of first recording in the Live TV chain.

- GetBufferEndTime()
  Returns minimum of current time and end time of last recording in the Live TV
  chain.