simulation: simplify using tgba_digraph more

* src/graph/graph.hh (new_states): New.
* src/tgba/tgbagraph.hh (graph_t): Make it public.
* src/tgbaalgos/simulation.cc: Get read of the acc_compl_automaton
class and replace it by a loop over all states of a tgba_digraph.
Remove some useless data structures.

src/tgba/tgbagraph.hh

@@ -149,8 +149,10 @@ namespace spot
 
   class tgba_digraph: public tgba
   {
-  protected:
+  public:
     typedef digraph<tgba_graph_state, tgba_graph_trans_data> graph_t;
+
+  protected:
     graph_t g_;
     bdd_dict* dict_;
     bdd all_acceptance_conditions_;