Update data: option to treat objects potentially deleted on the server as conflicts

[Gubaer]

#4509 reports problems with conflict resolution. This ​comment suggests an enhancement in the context of "Upload Data".

Current situation:

• run "Update Data". JOSM detects that there are n objects potentially deleted on the server.
• JOSM provides two options: ignore and skip check each potentially deleted object and delete it locally if it's indeed deleted on the server

Future situation:

• run "Update Data". JOSM detects that there are n objects potentially deleted on the server.
• JOSM provides three options: ignore and skip check each potentially deleted object and delete it locally if it's indeed deleted on the server check each potentially deleted object and create a conflict if the object is indeed deleted on the server. This allows JOSM users to treat objects deleted on the server individually.