Changesets should be split into smaller regions if possible

[ygra]

Right now you basically get a changeset covering the whole of France when editing a few buildings. That tends to annoy local mappers as such large-area changesets show up for everyone in that region, even though nothing has changed for most places. The random jumping to the next building anywhere else also contributes to the fact that smaller changesets would probably have to include only one building per changeset, so perhaps "next building" could also select something nearer to the last-answered one.