coremodel: Splice playlist model on changes

When reacting to the items-changed signal on the current playlist model,
all items were removed & added one at a time. This would result in
changes possibly stopping the playing playlist, as the playlist could
effectively become empty.

Instead make the signal handler smarter and splice the changes in
one go.

gnomemusic/coremodel.py

@@ -232,10 +232,7 @@ class CoreModel(GObject.GObject):
             return
 
         def _on_items_changed(model, position, removed, added):
-            if removed > 0:
-                for i in list(range(removed)):
-                    self._playlist_model.remove(position)
-
+            songs_list = []
             if added > 0:
                 for i in list(range(added)):
                     coresong = model[position + i]
@@ -243,8 +240,6 @@ class CoreModel(GObject.GObject):
                         coresong.props.media, self._coreselection,
                         self.props.grilo)
 
-                    self._playlist_model.insert(position + i, song)
-
                     song.bind_property(
                         "state", coresong, "state",
                         GObject.BindingFlags.SYNC_CREATE)
@@ -253,6 +248,10 @@ class CoreModel(GObject.GObject):
                         GObject.BindingFlags.BIDIRECTIONAL
                         | GObject.BindingFlags.SYNC_CREATE)
 
+                    songs_list.append(song)
+
+            self._playlist_model.splice(position, removed, songs_list)
+
         played_states = [SongWidget.State.PLAYING, SongWidget.State.PLAYED]
         for song in self._playlist_model:
             if song.props.state in played_states: