[mythtv-cmyth] Enabling 'Keep LiveTV Recording'

- Live timer doesn't create a recording rule anymore
Now we set recorder and keep recording. Backend will create rule as needed for us and no more recorder used.
- Add menuhook 'Keep LiveTV recording'
The improvement is extended to keep old live recordings using menuhook in recording screen
- cosmetics: MythDatabase::KeepLiveTVRecording()