Membership tabled in properties toggle dialog supports multiselect (and multiple membership deletion)
Property toggle dialog refactoring - methods splitting and reordering
see #7846: more RelationListDialog refactoring, all other relation-related actions separated from dialogs, @Override, JavaDocs

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5799 0c6e7542-c601-0410-84e7-c038aed88b3b