Selected option in semSearch dropdown may get stuck on non-preferred label after deep link

[jgonggrijp]

A very minor issue left after #495:

Select2 has the annoying property that if you select an <option>, it will no longer update its contents afterwards. Before 9c1ba6c, this resulted in chosen types and predicates in the semantic search dropdowns having blank labels when re-visiting the search form through a deep link. I mostly fixed it by adding more event handling steps, but due to our complex data model, the label of an option sometimes updates more than once. As a result, the Reader class may appear labeled as "Person" in this scenario.