[josm_plugins] update to JOSM 6906

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@30319 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4
[josm_roadsigns] remove duplicated images in ES preset (only differenciated in case by one letter => cannot have both versions under windows)

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@30320 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4