fix #8517 - sort validation results

This is accomplished by sorting the error list before constructing the tree.

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6468 0c6e7542-c601-0410-84e7-c038aed88b3b
Validation dialog: make `"x (y)"` strings translatable

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6469 0c6e7542-c601-0410-84e7-c038aed88b3b