ltl_to_tgba_fm: Build a tgba_digraph instead of a tgba_explicit_formula

The conversion is not complete, because the conversion from SERE to DRA
used for the closure operator is still building a tgba_explicit_formula.

* src/tgbaalgos/ltl2tgba_fm.cc, src/tgbaalgos/ltl2tgba_fm.hh: Return
a tgba_digraph.
* src/priv/acccompl.cc: Simplify.
* src/graph/ngraph.hh: Add a way to iterate over all names.
* src/tgba/tgbagraph.hh (compute_support_conditions): Return something
useful.  It's actually used by the constructor of testing automata.
* src/tgbatest/wdba.test: Adjust to the fact that state are not
labeled by formulas anymore.
* src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc: Do not try to enable
UTF8 on automata anymore.

src/graph/ngraph.hh

@@ -94,6 +94,16 @@ namespace spot
       return state_to_name.at(s);
     }
 
+    bool has_state(name n) const
+    {
+      return name_to_state.find(n) != name_to_state.end();
+    }
+
+    state_to_name_t& names()
+    {
+      return state_to_name;
+    }
+
     template <typename... Args>
     transition
     new_transition(name src, name dst, Args&&... args)