Backup old plugins when updating them

[rickmastfan67]

This enhancement idea comes from the problem I experienced because of #6501.

If you update your plugins in JOSM, JOSM should backup them into a "plugins/backup" (or "plugins_backup") folder in case there is a major problem that prevents you from getting JOSM to start with the newest version of the plugins. All you would then have to do is copy the versions out of the "plugins/backup" folder and replace the bad versions to allow JOSM to start again.

This would be a lot better than having to hunt around on the OSM Trac just to find the last version of the plugins that work in JOSM and download them. Would save a lot of time and annoyance on the part of the end user.