bin: new ltlmix tool

Fixes #400.

* spot/tl/randomltl.cc, spot/tl/randomltl.hh: Adjust to accept
a set of formula to replace the atomic propositions.
* bin/ltlmix.cc: New file.
* bin/Makefile.am: Add it.
* bin/man/ltlmix.x: New file.
* bin/man/Makefile.am: Add it.
* doc/org/ltlmix.org: New file.
* doc/Makefile.am: Add it.
* bin/man/genltl.x, bin/man/randltl.x, bin/man/spot.x, bin/spot.cc,
doc/org/arch.tex, doc/org/concepts.org, doc/org/tools.org, NEWS: Mention
ltlmix.
* tests/core/ltlmix.test: New file.
* tests/Makefile.am: Add it.

NEWS

@@ -2,6 +2,9 @@ New in spot 2.12.0.dev (not yet released)
 
   Command-line tools:
 
+  - ltlmix is a new tool that generate formulas by combining existing
+    ones. See https://spot.lre.epita.fr/ltlmix.html for examples.
+
   - autfilt learned --restrict-dead-end-edges, to restricts labels of
     edges leading to dead-ends.  See the description of
     restrict_dead_end_edges_here() below.
@@ -38,8 +41,8 @@ New in spot 2.12.0.dev (not yet released)
 
   Bug fixes:
 
-  - Generating random formula without any unary opertors would very
-    often create formulas much smaller than asked.
+  - Generating random formulas without any unary opertor would very
+    often create formulas much smaller than specified.
 
 New in spot 2.12  (2024-05-16)