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.

tests/core/complete.test

@@ -1,6 +1,6 @@
 #! /bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2015-2017 Laboratoire de Recherche et Développement
+# Copyright (C) 2015-2017, 2022 Laboratoire de Recherche et Développement
 # de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -236,6 +236,7 @@ properties: deterministic univ-branch
 State: 0 {0}
 [t] 0
 State: 1 {0}
+[f] 1
 [t] 0
 --END--
 EOF
@@ -272,7 +273,8 @@ properties: trans-labels explicit-labels state-acc deterministic
 properties: univ-branch
 --BODY--
 State: 0
-State: 1
+State: 1 {0}
+[f] 1
 --END--
 EOF
 diff out expected