The LibraryBackend has now been split into a Database class that deals with setting up sqlite, and PlaylistBackend that contains the functions for persisting the playlist. The LibraryBackend now only contains functions for accessing "a collection of songs", and can be parameterised with table names to access different collections. It also no longer lives in a background thread, and follows the Qt memory management model instead of using shared_ptr.
Most of Library has been moved into LibraryModel - a QAbstractItemModel for any LibraryBackend. What's left of Library is now specific to the user's local library on disk.
David Sansome