dot: fix printing of alternating automata

Related to #208.

* spot/twaalgos/dot.cc: Fix missing definitions of universal nodes,
and inclusion of universal nodes inside of SCC when none of the
destination comes back to the SCC.
* tests/python/_altscc.ipynb: Adjust and add more test cases.
* tests/core/alternating.test, tests/core/neverclaimread.test,
tests/core/readsave.test, tests/core/sccdot.test,
tests/python/decompose.ipynb: Adjust test cases.
* NEWS: Mention the bug.

tests/core/readsave.test

@@ -327,21 +327,21 @@ digraph G {
   subgraph cluster_0 {
   color=green
   1 [label="s1", peripheries=2]
-  1 -> 1 [label="a"]
   }
   subgraph cluster_1 {
   color=green
   0 [label="s0", peripheries=2]
-  0 -> 0 [label="b"]
   }
   subgraph cluster_2 {
   color=black
   3 [label="s3"]
-  3 -> 1 [label="a"]
-  3 -> 0 [label="b"]
   }
+  0 -> 0 [label="b"]
+  1 -> 1 [label="a"]
   2 [label="s2"]
   2 -> 0 [label="b"]
+  3 -> 1 [label="a"]
+  3 -> 0 [label="b"]
 }
 EOF