Asset service to allow filtering by asset group

Default to enumerating all asset groups
Enumerate assets only from saved photos.

Also fix bug causing asset library enumeration to be skipped (attempted to
start enumeration before setting up SSChronologicalAssetLibraryService)
Fix typo causing last photo to be duplicated in viewer
Merge branch 'issues/asset-library-photo-tracking-35' into develop

Fixes #35