Notable changes:

  * User data (favourites, albums) is now stored in a private database
    in ~/.local/share/gnome-photos. This is combined with the
    filesystem miner's index of items at query time.

  * Inside Flatpak, the application connects to Tracker via the new
    xdg-tracker-portal instead of talking directly over D-Bus. Access
    is limited by the portal so the app can only see the Pictures
    graph.

  * The Flatpak build can use a private instance of Tracker 3's
    filesystem miner when running on hosts without Tracker 3.

Currently, nfo:fileName is absent from the tracker:Pictures graph [1],
which leads to the following WARNING:
  gnome-photos-WARNING **: (../src/photos-local-item.c:91):
    photos_local_item_create_filename_fallback: code should not be
    reached

Similarly, the built-in nfo:image-category-screenshot collection is
also missing.

[1] tracker#221

#152