Emitted when one or more Individuals are added to or removed from the view.
The sets of `added` and `removed` individuals are sorted by descending match strength. Using the Gee.SortedSet.lower and Gee.SortedSet.higher APIs with SearchView.individuals, the `added` individuals can be inserted at the correct positions in a UI representation of the search view.
a set of Individuals added to the search view
a set of Individuals removed from the search view