[Patch awaiting end of stabilization phase] Warn when deleting a large relation

[Mormegil]

When the user tries to delete a large relation (more than, say, 10–50 members), JOSM should warn him and ask if he really intends to do that. I have spent the better part of this weekend restoring relations for European E-roads (see ​WikiProject Europe/E-road network) accidentally deleted by various users.

Maybe a warning could be issued always when deleting a relation, no matter its size, since that is quite a rare operation, but I am not sure about that. But since undeleting a large relation (E-road relations have hundreds and thousands of members) is really painstaking, I believe another layer of precaution would be a good idea.