improve support for LTLf semantics

* spot/twaalgos/remprop.cc, spot/twaalgos/remprop.hh (to_finite): New
function.
* bin/autfilt.cc (--to-finite): Add it.
* doc/org/tut12.org: Update to use it.
* spot/twa/twagraph.cc (purge_dead_states): Also remove false edges.
* spot/parseaut/parseaut.yy: Do not ignore false self-loops, and
add false self-loop on accepting states without successors.
* NEWS: List the above changes.
* tests/core/ltlf.test: New file.
* tests/Makefile.am: Add it.
* tests/core/complete.test: Adjust expected output.

NEWS

@@ -6,6 +6,9 @@ New in spot 2.10.4.dev  (net yet released)
     if the output code should attempt to preserve aliases
     present in the HOA input.  This defaults to "keep".
 
+  - autfilt has a new --to-finite option, illustrated on
+    https://spot.lrde.epita.fr/tut12.html
+
   Library:
 
   - "original-classes" is a new named property similar to
@@ -39,6 +42,21 @@ New in spot 2.10.4.dev  (net yet released)
   - print_dot() learned option "@" to display aliases, as discussed
     above.
 
+  - to_finite() is a new function that help interpreting automata
+    build from LTLf formula using the from_ltlf() function.  It replace
+    the previously suggested method of removing and atomic proposition
+    and simpifying automata, that failed to deal with states without
+    successors.  See updated https://spot.lrde.epita.fr/tut12.html
+
+  - the HOA parser learned to not ignore self-loops labeled with [f]
+    and to turn any state that have colors but no outgoing transitions
+    into a state with a [f] self-loop.  This helps dealing with
+    automata containing states without successors, as in the output of
+    to_finite().
+
+  - purge_dead_states() will now also remove edges labeled by false
+    (except self-loops).
+
 New in spot 2.10.4  (2022-02-01)
 
   Bug fixed: