[mythtv-cmyth] Add MythConnection::GenerateRecordingPreview

This function query backend to generate a preview for handle recording.
[mythtv-cmyth] Add MythDatabase::KeepLiveTVRecording

This function query backend to disables/enables auto expiration and sets the storage group to 'Default'/'LiveTV' to keep/release handle recording.
[mythtv-cmyth] Add MythProgramInfo::StorageGroup

The function returns storage group of recording.
[mythtv-cmyth] Resolving confusion between 'Recording Group' and 'Storage Group'

Storage group is mistakenly confused with recording group because they contain similar values.
Files, like previews, icons, artworks and streams, should be reached for a storage group.
[mythtv-cmyth] Assign RecordingStartTime to PVR_RECORDING.recordingTime

PVR seems to identify recording with key {title,recordingTime}. Using start time of EPG program make confusion between recordings of the same EPG. Assign RecordingStartTime to recordingTime instead.
[mythtv-cmyth] Keeping live recordings doesn't update recording list

This case occurs on keeping a live recording and stopping watching: MythTV create a new rule (recordid) for the recording, which is part of our UID. So replace the old live recording without rule (recordid=0) by this one.
[mythtv-cmyth] Add KeepLiveTVrecording (frodo)

The function implements the pvr operation "keep live recording". We can keep watching recording or old not deleted. A new GUI dialog allow to select recording from list. Once selected by the user 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 new action is available from channel screen.