Simplified iterators and made them integrate better with the STL.

Specifically, value_type has changed from {Vertex,Edge,...} to
{Vertex,Edge,...}Handle so that dereferenced iterators can actually be put to
use, now.


git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@547 fdac6126-5c0c-442c-9429-916003d36597