[mythtv] Allow reset of customized flags and properties from ProgramInfo

- New member Reset() to reset m_flags and m_props
- Cosmetics in ProgramInfo class
[cppmyth] GetRecordingArtworkList

- Add GetRecordingArtworkList()  
- Filling or refreshing Artwork for Program initialized by proto or wsapi
Program provided by proto hasn't populated artwork. RefreshRecordedArtwork() fill artwork 
of an existing Program.
[mythtv] Fix refreshing artwork after program update event