rank nodes
====================
The initial state should put at the top.

See DCWorkflowGraph_Rank.py

shorter title
====================
I found the transition title is quite long in this version. This makes
the graph a bit ugly.

I want to show the transition id only and make the full title visible
only when hover on the transition id.

I have some idea to make hover works:

- in the pot file, give each state and transition a URL attribute,
this will generate links in the graph

- use javascript to register onhover event listener and show the title
at some place in the page.