Add an index to the filter, returning an integer that can be used to identify the index in the selection functions.

An integer that can be used in selectItems(), selectAll(), etc., to identify this index in the SearchFilter. If the index is already present in the search filter, the index is not added a second time, and the identifier for the existing instance of the index is returned. Therefore, addIndex() can be used to get the identifier for an index that has previously been added to the SearchFilter.