Removed slow search for existing selectionlist items

When the selectionlist is filled with more than just a few items (445 artists in my case) they appear really slowly in the UI. This is caused by a slow search for existing items. In update_values there are len(self._model) * len(values) comparisons. This method also calls add_value which looks for existing items again. So 2 * len(self._model) * len(values) comparisons in total.

My changes instead create a set of item ids and look up ids of new items there. I also removed the unnecessary thread for populate and simplified add_values.