added a set method to observable_list
observable_list.remove: no longer trigger removed event on the element itself

-> code relying on such functionality needs to be adapted to delegate the removed event of the list to the removed element
observable_list refactor:

- extracted superclass object_list that provides a list with event support for plain objects
- observable_list is now subclass of object_list and uses observables internally (like before)
- constructor parameter modifier has been removed
- schnipp.models.selectable_list_refinement adds support to track a selected element of the list
refactored ui.SingleSelectListView

- extracted _handle_click method
- renamed select_element to _update_view_selection