[mythtv-cmyth] Use term 'Storage Group' when loading files

The current code confuses the term recording group and storage group
when loading files.

Files (like previews icons, artworks and streams) are accessible by their
storage group.

Whereas recording groups are used to set user-defined categories for
recordings (like Movies, Kids, ...).
[mythtv-cmyth] Add MythProgramInfo::StorageGroup

The function returns the recording's storage group.
[mythtv-cmyth] Use StorageGroup instead of RecordingGroup when loading preview

Prevents mythbackend errors like:
  Unable to find any directories for the local storage group 'Movies' on
    'myhost', trying directories on all hosts!
  Unable to find storage group 'Movies', trying 'Default' group!