[dvbviewer] pump version to 1.6.15
[dvbviewer] add genre
[dvbviewer] minor code cleanup
[dvbviewer] don't cache recordings
[dvbviewer] cleanup timeshift api a bit

Allow fetching the TS buffer instead of wrapping all calls through
dvbdata. Additional implement GetBufferTimeStart, GetBufferTimeEnd
and GetPlayingTime.
[dvbviewer] minor cleanups in GenerateTimer
[dvbviewer] speed up timeshifting start