#7665 - merge conflation operations for multiple pairs into one undo-able action

[don-vip]

From https://josm.openstreetmap.de/ticket/7665:

At the moment each conflation operation is added as a single operation to the undo stack, even when conflating a lot of pairs at once.
All core operations add a single atomic operation to the undo stack when manipulating many objects, so this isn't consistent.