fix #8512 - Do not display discardable keys by default. Expert users can display them by enabling "Display discardable keys" in OSM Data preferences, and change their display colour

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6314 0c6e7542-c601-0410-84e7-c038aed88b3b
Merge branch 'master' into mirror