coremodel: Correctly register playlists availability
In some cases (especially on the flatpak version), the playlists model is populated before the PlaylistsView is created. The "playlists-loaded" signal is never received by the view because it has already been emitted. This results in a broken view.
This issue can be fixed by replacing the playlists-loaded
signal with
a playlists_available
property. Indeed, when MPRIS or PlaylistsView
need to know if the playlists are loaded, they can check the
playlists_available
value. If False, they need to register to
notify::playlists-available
signal to wait for the availability.
Closes: #348 (closed)