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.

tests/Makefile.am

@@ -176,6 +176,7 @@ TESTS_tl = \
   core/kind.test \
   core/remove_x.test \
   core/ltlrel.test \
+  core/ltlmix.test \
   core/ltlgrind.test \
   core/ltlcrossgrind.test \
   core/ltlfilt.test \