Improve Remove function

[nkamapper]

The Conflation plugin has proven very useful when importing POI type datasets, so I hope it is still being maintained.

As expected, the Remove button currently excludes a matched pair of subject/reference items from the Matches list. However, both the subject item and the reference item are then gone from the selection, never to be found again. In the case of an import, that means that one item (the reference) is lost. It would be better, I think, if the Remove button moved the subject/reference pair from the Matches list to the Subject Only list and to the Reference Only list, respectively.