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
2 jobs for wip/jfelder/coremodel-playlists-available in 9 minutes and 36 seconds