Fixed constness of EntityRange begin() and end() methods.

git-svn-id: http://openmesh.org/svnrepo/OpenMesh@1182 fdac6126-5c0c-442c-9429-916003d36597
PropertyManager: Added static copy(...) convenience method.

git-svn-id: http://openmesh.org/svnrepo/OpenMesh@1183 fdac6126-5c0c-442c-9429-916003d36597