CollectionModels: Use tracks function to return items for collection

SharedSources/MediaLibraryModel/ArtistModel.swift

@@ -92,6 +92,6 @@ extension VLCMLArtist: MediaCollectionModel {
     }
 
     func files() -> [VLCMLMedia] {
-        return []
+        return tracks()
     }
 }

SharedSources/MediaLibraryModel/GenreModel.swift

@@ -104,6 +104,6 @@ extension VLCMLGenre: MediaCollectionModel {
     }
 
     func files() -> [VLCMLMedia] {
-        return []
+        return tracks()
     }
 }