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/misc/escape.cc

@@ -77,6 +77,31 @@ namespace spot
     return os;
   }
 
+  std::ostream&
+  escape_html(std::ostream& os, const std::string& str)
+  {
+    for (auto i: str)
+      switch (i)
+	{
+	case '&':
+	  os << "&amp;";
+	  break;
+	case '"':
+	  os << "&quot;";
+	  break;
+	case '<':
+	  os << "&lt;";
+	  break;
+	case '>':
+	  os << "&gt;";
+	  break;
+	default:
+	  os << i;
+	  break;
+	}
+    return os;
+  }
+
   std::ostream&
   escape_str(std::ostream& os, const std::string& str)
   {

src/misc/escape.hh

@@ -46,6 +46,13 @@ namespace spot
   SPOT_API std::ostream&
   escape_latex(std::ostream& os, const std::string& str);
 
+  /// \brief Escape special HTML characters.
+  ///
+  /// The following characters are rewritten:
+  /// <code>&gt; &lt; &quot; &amp;</code>
+  SPOT_API std::ostream&
+  escape_html(std::ostream& os, const std::string& str);
+
   /// \brief Escape characters <code>"</code>, <code>\\</code>, and
   /// <code>\\n</code> in \a str.
   SPOT_API std::ostream&