smartplaylist: Add missing playlist-loaded emission
There are two ways to start playing a playlist:
- from PlaylistsView, by clicking on a song
- from a MPRIS client which handles the MediaPlayer2.Playlists interface
In the second case, the playlist model has not been loaded yet if it has not been displayed in the PlaylistsView (models are loaded on demand). In that case, it is necessary to register to the "playlist-loaded" signal to the set player model once the playlist model is loaded. It works correctly for user playlists but not for smart playlists. Indeed, the latter do not emit the "playlist-loaded" signal.
Fix this issue by emitting the "playlist-loaded" signal once a smart playlist model is loaded.