[mythtv-cmyth] Build demuxer and use it in the addon

New setting 'demuxing' in category 'advanced' allows to enable/disable the fonctionality.
[mythtv-cmyth] Implements timeshift functions for PVR API 1.9.0

- GetPlayingTime()
  Available only using internal demuxing MPEG-TS, else default value will be returned to PVR.

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

- GetBufferEndTime()
  Returns least of current time and end time of last recording in the liveTV chain.