Change Avalonia's Task-based approach to WinForms' BackgroundWorker approach.
- Reduce number of calls to GetLibrary by adding the Library to the LibraryStats record.
There are multiple subscribers to LibraryCommands.LibrarySizeChanged, and each one calls GetLibrary_Flat_NoTracking(). Passing the full library as an event argument speeds up all operations which happen after the library size changes.
Fix initial backup counts