fix #20325: Update Multipolygon removes tags instead of moving them to relation
rewrite handling of update multipolygon cases
let removeTagsFromWaysIfNeeded() check if getDataset() returns null instead of checking isNew(). I assume it was always meant to work like this. JoinAreasAction works fine with that and I hope no plugin relies on the old behaviour.
add regression unit test and more unit tests to improve coverage