Clean up dotty output of TAs.

* src/taalgos/dotty.cc: Clean up output of TAs.
* src/tgbatest/ltl2tgba.cc: Fix memory management, and use the TA
printer for TGTA.
* wrap/python/spot.i (as_ta): New function to convert a tgta_explicit
into a TA.
* wrap/python/ajax/spot.in: Use this new function to display automata.

wrap/python/spot.i

@@ -317,6 +317,14 @@ using namespace spot;
 
 }
 
+%extend spot::tgta_explicit {
+  const spot::ta*
+  as_ta(const spot::tgta_explicit* t)
+  {
+    return dynamic_cast<const spot::ta*>(t);
+  }
+}
+
 %nodefaultctor std::ostream;
 namespace std {
   class ostream {};