dot: replace large labels by "(label too long)"

Based on a report by Victor Khomenko.

* spot/twaalgos/dot.cc: Here.
* tests/core/readsave.test: Add test case.
* NEWS: Mention it.

tests/core/readsave.test

@@ -1104,3 +1104,8 @@ digraph "a U b" {
 }
 EOF
 diff out expected
+
+f="{{!a;!b}:{{c <-> d} && {e xor f} && {m | {l && {k | {j <-> {i xor {g && h}"
+f="$f}}}}} && {{n && o} | {!n && p}} && {q -> {r <-> s}}}:{[*0..1];t}}[]-> u"
+ltl2tgba -f "$f" --dot=bar > out.dot
+grep 'label too long' out.dot