Emit dataChanged when changing skip track state.

Otherwise, when changing this for the currently playing track, the "strike out" change wasn't visible.

src/playlist/playlist.cpp

@@ -2142,5 +2142,6 @@ void Playlist::SkipTracks(const QModelIndexList& source_indexes) {
   for (const QModelIndex& source_index : source_indexes) {
     PlaylistItemPtr track_to_skip = item_at(source_index.row());
     track_to_skip->SetShouldSkip(!((track_to_skip)->GetShouldSkip()));
+    emit dataChanged(source_index, source_index);
   }
 }