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.

src/tgba/acc.hh

@@ -660,6 +660,24 @@ namespace spot
       // Return the set of sets appearing in the condition.
       acc_cond::mark_t used_sets() const;
 
+      // Return the sets used as Inf or Fin in the acceptance condition
+      std::pair<acc_cond::mark_t, acc_cond::mark_t> used_inf_fin_sets() const;
+
+      // Print the acceptance as HTML.  The set_printer function can
+      // be used to implement customized output for set numbers.
+      std::ostream&
+      to_html(std::ostream& os,
+	      std::function<void(std::ostream&, int)>
+	      set_printer = nullptr) const;
+
+      // Print the acceptance as text.  The set_printer function can
+      // be used to implement customized output for set numbers.
+      std::ostream&
+      to_text(std::ostream& os,
+	      std::function<void(std::ostream&, int)>
+	      set_printer = nullptr) const;
+
+      // Calls to_text
       SPOT_API
       friend std::ostream& operator<<(std::ostream& os, const acc_code& code);
     };