dotty: colored acceptance sets

This implement several new options for --dot in order to
allow emptiness sets to be output as colored ⓿ or ❶...
Also add a SPOT_DOTDEFAULT environment variable.

* NEWS, src/bin/man/spot-x.x, src/bin/common_aoutput.cc,
src/bin/dstar2tgba.cc: Document the new options.
* doc/org/.dir-locals.el, doc/org/init.el.in: Setup
SPOT_DOTEXTRA and SPOT_DOTDEFAULT for all documents.
* doc/org/autfilt.org, doc/org/dstar2tgba.org, doc/org/ltl2tgba.org,
doc/org/ltldo.org, doc/org/oaut.org, doc/org/randaut.org,
doc/org/satmin.org: Adjust to this new setup.
* src/misc/escape.cc, src/misc/escape.hh (escape_html): New function.
* src/tgba/acc.cc, src/tgba/acc.hh (to_text, to_html): New method.
* src/tgbaalgos/dotty.cc: Implement the new options.
* src/tgbatest/readsave.test, wrap/python/tests/automata.ipynb: More
tests.
* wrap/python/spot.py: Make sure the default argument for
dotty_reachable is None, so that SPOT_DOTDEFAULT is honored.

doc/org/init.el.in

@@ -24,6 +24,9 @@
 (setq org-babel-sh-command
       (concat "PATH=@abs_top_builddir@/src/bin" path-separator "$PATH sh"))
 
+(setenv "SPOT_DOTDEFAULT" "brf(Lato)")
+(setenv "SPOT_DOTEXTRA" "node[style=filled, fillcolor=\"#ffffa0\"]")
+
 (setq org-export-html-home/up-format
 "<div id=\"org-div-home-and-up\" style=\"text-align:center;white-space:nowrap;\">
  <a accesskey=\"h\" href=\"%s\">UP</a> | <a accesskey=\"H\" href=\"%s\">HOME</a>