graph: store the source indices in the transition vector

... and use it to sort transitions.

* src/graph/graph.hh: Adjust storage of source index.  Provide
remove_dead_transitions_(), sort_transitions_() and
chain_transitions_() methods.
* src/tgba/tgbagraph.cc (merge_transitions): Rewrite using
above methods.
* src/tgba/tgbagraph.hh: Add a comparison operator for
transitions.
* src/tgbatest/degenlskip.test, src/tgbatest/det.test,
src/tgbatest/ltl2ta.test, src/tgbatest/neverclaimread.test,
src/tgbatest/readsave.test: Adjust expected transition order in test
cases.

src/tgba/tgbagraph.hh

@@ -87,6 +87,15 @@ namespace spot
       : cond(cond), acc(acc)
     {
     }
+
+    bool operator<(const tgba_graph_trans_data& other) const
+    {
+      if (cond.id() < other.cond.id())
+	return true;
+      if (cond.id() > other.cond.id())
+	return false;
+      return acc < other.acc;
+    }
   };