twa_graph: add a merge_univ_dests() method

and call it after parsing

* spot/twa/twagraph.cc, spot/twa/twagraph.hh
(twa_graph::merge_univ_dests): New method.
* spot/parseaut/parseaut.yy: Call it.
* spot/twaalgos/dot.cc: Improve output, now that
several edges can use the same universal destination.
* tests/core/alternating.test, tests/core/complete.test,
tests/core/parseaut.test, tests/python/_altscc.ipynb,
tests/python/alternating.py, tests/python/alternation.ipynb: Adjust
test case.
* doc/org/tut24.org: Adjust example.

tests/core/complete.test

@@ -203,7 +203,7 @@ properties: deterministic very-weak
 --BODY--
 State: 0
 [!0 | 1] 0
-[0&!1] 1&0
+[0&!1] 0&1
 State: 1 {0}
 [0&!1] 1
 [!0 | 1] 1