see #8987 - add missing clone() methods to fix broken plugins

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6167 0c6e7542-c601-0410-84e7-c038aed88b3b
see #8987 - make it public

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6168 0c6e7542-c601-0410-84e7-c038aed88b3b