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.

doc/org/concepts.org

@@ -158,7 +158,7 @@ $txt
 #+END_SRC
 
 #+RESULTS:
-[[file:concept-buchi.svg]]
+[[file:concept-buchi2.svg]]
 
 The =1= displayed on the edge that loops on state =1= should be
 read as /true/, i.e., the Boolean formula that accepts