CollectionModel: Add removal from Playlist

(fixes #507)

SharedSources/MediaLibraryModel/CollectionModel.swift

@@ -40,7 +40,13 @@ class CollectionModel: MLBaseModel {
     var indicatorName: String = "Collections"
 
     func delete(_ items: [VLCMLObject]) {
-       assertionFailure("still needs implementation")
+        if let playlist = mediaCollection as? VLCMLPlaylist {
+            for case let media as VLCMLMedia in items {
+                if let index = files.firstIndex(of: media) {
+                    playlist.removeMedia(fromPosition: UInt32(index))
+                }
+            }
+        }
     }
 }