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.

NEWS

@@ -110,9 +110,11 @@ New in spot 1.99b (not yet released)
     - GraphViz output now uses an horizontal layout by default.  The
       --dot option of the various command-line tools takes an optional
       parameter to fine-tune the GraphViz output (including vertical
-      layout, round states, and named automata).  The environment
-      variable SPOT_DOTEXTRA can also be used to add extra attributes
-      to the output graph.
+      layout, round states, named automata, and different ways to
+      colorize the acceptance sets).  The environment
+      variables SPOT_DOTDEFAULT and SPOT_DOTEXTRA can also be used
+      to respectively provide a default argument to --dot, and
+      add extra attributes to the output graph.
 
     - Never claims can now be output in the style usd by Spin since
       version 6.2.4 (i.e., using do..od instead of if..fi, and with