[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' (frodo)

The action is available from channel screen.
[mythtv-cmyth] Add notification 'Keeping LiveTV recording'