Kompare: Finally add support for refreshing diffs to the trunk.
(Though it was possible to get that effect with the "swap and swap back" hack which I got all too accustomed to. ;-) )

Partly based on revision 418173 by jkeel from 3_way_kompare, also incorporates my followup (revision 871142), the actual implementation of the refreshing reimplemented to work in trunk.