* spot/twaalgos/dot.cc: Improve the aiger printer by using a more
traditional dot syntax, indenting the output, adding some hard-coded
colors, fixing a bug in the negation of latch inputs, and rotating the
triangles for horizontal output.
* tests/python/synthesis.ipynb: Adjust expected output, and add
an example of horizontal layout.
* spot/twaalgos/dot.cc (print_dot): Add some detection of Mealy
machines, and some code to render its I/O in a <table>.
* tests/python/synthesis.ipynb: Adjust expected output.
* tests/python/_mealy.ipynb: New file.
* tests/Makefile.am: Add python/_mealy.ipynb.
* NEWS: Mention the new feature.
