implement is_liveness() and is_liveness_automaton()

* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh,
spot/tl/hierarchy.cc, spot/tl/hierarchy.hh: Here.
* bin/ltlfilt.cc (--liveness): New filter.
* NEWS: Mention those.
* tests/core/ltlfilt.test, tests/python/ltlsimple.py: Add test cases.

NEWS

@@ -15,11 +15,8 @@ New in spot 2.6.2.dev (not yet released)
     Zielonka algorithm. Calude's quasi-polynomial time algorithm has
     been dropped as it was not used.
 
-  Build:
-
-  - We no longer distribute the Python-based CGI script + javascript
-    code for the online translator.  Its replacement has its own
-    repository: https://gitlab.lrde.epita.fr/spot/spot-web-app/
+  - ltlfilt learned --liveness to match formulas representing liveness
+    properties.
 
   Library:
 
@@ -64,6 +61,10 @@ New in spot 2.6.2.dev (not yet released)
     with arbitrary acceptance condition into a parity automaton,
     based on a last-appearance record (LAR) construction.
 
+  - The new function is_liveness() and is_liveness_automaton() can be
+    used to check whether a formula or an automaton represents a
+    liveness property.
+
   Bugs fixed:
 
   - Running "ltl2tgba -B" on formulas of the type FG(safety) would