Add mName member to aiMesh.

JoinVerticesProcess and SortByPType copy the mesh name to all clone or sub meshes they create.
JoinVerticesProcess' verbose output now prints mesh name, no longer using sprintf (TinyFormatter's age has come).

WARN: This is a public API change (not a breaking one, however).

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@719 67173fc5-114c-0410-ac8e-9d2fd5bffc1f