[mythtv-cmyth] Keep LiveTV recording (menu hook action)

- PVR seems to identify recording with tuple(title,recordingTime,...?). Using start time of EPG program make confusion between recordings of the same EPG. Assign RecordingStartTime to recordingTime instead.
- Use StorageGroup() instead RecordingGroup() for fileOps.
- Add new case "Replace recording" in EventUpdateRecording when linking live recording with a rule.
- New menu hook for the channel category: action is "Keep LiveTV recording", ID=3.
- Adding new dialog to select recording from list. Action "Keep LiveTV recording" open the dialog to select the liveTV recording to keep.