[mythtv-cmyth] Add MythProgramInfo::IsLiveTV
[mythtv-cmyth] Add group 'LiveTV' in recordings view

Adding group we can see LiveTV recordings and later doing specific actions.
Recordings with duration less than 5s still not visible to not showing dummy recordings.
[mythtv-cmyth] Add KeepLiveTVrecording

The function implements the pvr operation "keep live recording". We can keep watching recording or old not deleted. The function call the backend operation, refresh the recording list and query to generate the preview.
[mythtv-cmyth] Add menuhook 'Keep LiveTV recording'

The new action is available from recordings screen.