introduce op::strong_X

This was prompted by reports by Andrew Wells and Yong Li.

* NEWS, doc/tl/tl.tex: Document the changes.
* THANKS: Add Andrew.
* bin/ltlfilt.cc: Match --ltl before --from-ltlf if needed.
* spot/parsetl/parsedecl.hh, spot/parsetl/parsetl.yy,
spot/parsetl/scantl.ll: Parse X[!].
* spot/tl/formula.cc, spot/tl/formula.hh: Declare the new operator.
* spot/tl/ltlf.cc: Adjust to handle op::X and op::strong_X correctly.
* spot/tl/dot.cc, spot/tl/mark.cc, spot/tl/mutation.cc,
spot/tl/print.cc, spot/tl/simplify.cc, spot/tl/snf.cc,
spot/tl/unabbrev.cc, spot/twa/formula2bdd.cc,
spot/twaalgos/ltl2taa.cc, spot/twaalgos/ltl2tgba_fm.cc,
tests/core/ltlgrind.test, tests/core/rand.test,
tests/core/sugar.test, tests/python/randltl.ipynb: Adjust.
* tests/core/ltlfilt.test, tests/core/sugar.test,
tests/core/utf8.test: More tests.

tests/core/rand.test

@@ -114,17 +114,19 @@ grep -q p2 out
 grep p3 out && exit 1
 
 
-# We should be able to generate exactly two formulas with 0 atomic
-# propositions.
-run 0 randltl -n2 0 | sort > out
+# We should be able to generate exactly three formulas with 0 atomic
+# propositions and at most 2 nodes.
+run 0 randltl -n3 --tree-size=..2 0 > out
+sort <out >out2
 cat >expected <<EOF
 0
 1
+X(0)
 EOF
-diff out expected
+diff out2 expected
 
 # requesting more formulas should fail
-run 2 randltl -n3 0
+run 2 randltl -n4 --tree-size=..2 0
 
 # If more numbers are given, there are interpreted as atomic propositions
 run 0 randltl -n1000 0 1 > out