Notable changes:

  * User data (favourites, albums) is now stored in a private database in
    ~/.local/share/gnome-photos. This is combined with the Tracker Miner
    FS index of photos at query time.
  * Inside Flatpak, the app 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 bundled version of Tracker Miners, if a
    suitable version is not available on the host.
  * Change detection is done using TrackerNotifier instead of watching
    the GraphUpdated D-Bus signal directly.

Closes #59
Closes #152