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.

wrap/python/spot.py

@@ -31,7 +31,7 @@ def _ostream_to_svg(ostr):
     res = dotty.communicate()
     return res[0].decode('utf-8')
 
-def _render_automaton_as_svg(a, opt=""):
+def _render_automaton_as_svg(a, opt=None):
     ostr = ostringstream()
     dotty_reachable(ostr, a, opt)
     return _ostream_to_svg(ostr)
@@ -47,7 +47,7 @@ def _render_formula_as_svg(a):
     dotty(ostr, a)
     return SVG(_ostream_to_svg(ostr))
 
-def _render_tgba_as_svg(a, opt=""):
+def _render_tgba_as_svg(a, opt=None):
     # Load the SVG function only if we need it. This way the bindings
     # can still be used outside of IPython if IPython is not
     # installed.