tl: extend F[n:m] and G[n:m] to the case of m=$

Suggested by Victor Khomenko.

* spot/tl/formula.cc, spot/tl/formula.hh, spot/parsetl/parsetl.yy:
Implement this.
* NEWS, doc/tl/tl.tex: Document it.
* tests/core/sugar.test, tests/python/ltlparse.py: Add some tests.

NEWS

@@ -93,6 +93,9 @@ New in spot 2.7.4.dev (not yet released)
     terms of the existing PSL operators.  ##[+] and ##[*] are sugar
     for ##[1:$] and ##[0:$].
 
+  - The F[n:m] and G[n:m] operators introduced in Spot 2.7 now
+    support the case where m=$.
+
   - spot::relabel_apply() makes it easier to reverse the effect
     of spot::relabel() or spot::relabel_bse() on formula.