add a --check=stutter-sensitive-example option

* spot/twaalgos/stutter.cc,
spot/twaalgos/stutter.hh (check_stutter_invariance): Add a
find_counterexamples argument.
* spot/twaalgos/hoa.cc: Output accepted-word and rejected-word examples.
* bin/common_aoutput.cc: Handle --check=stutter-sensitive-example.
* NEWS: Mention it.
* tests/core/stutter-tgba.test: Test it.
* doc/org/concepts.org, doc/org/hoa.org: Document accepted-word and
rejected-word named properties.
* bin/man/spot-x.x: Mention that --check=stutter-sensitive-example
ignores SPOT_STUTTER_CHECK.

NEWS

@@ -1,5 +1,19 @@
 New in spot 2.8.3.dev  (not yet released)
 
+  Command-line tools:
+
+  - When the --check=stutter-sensitive-example option is passed to
+    tools like ltl2tgba, autfilt, genaut, or ltldo, the produced
+    automata are checked for stutter-invariance (as in the
+    --check=stutter-invariant case), additionally a proof of
+    stutter-sensitiveness is provided as two stutter-equivalent words:
+    one accepted, and one rejected.  These sample words are printed in
+    the HOA output.
+
+    % ltl2tgba --check=stutter-sensitive-example Xa | grep word:
+    spot-accepted-word: "!a; cycle{a}"
+    spot-rejected-word: "!a; !a; cycle{a}"
+
   Library:
 
   - Historically, Spot only supports LTL with infinite semantics