Highlight unused will not be reset

[Murmele]

Git master: a75565e

1. Create new Project and add two different elements
2. Save the project and close it
3. Reopen the project
4. Remove on element and save
   --> in Collections the element should get a red background, because it is unused
5. Add the element with the red background and save
   --> Collections should update and the element should get a white background again.

I think for that in
void ElementsCollectionModel::highlightUnusedElement()
the elements which are not unused should be reset to a white background?