dot: add x option for dot2tex

* spot/twa/acc.cc, spot/twa/acc.hh: Add a LaTeX output for acceptance
conditions.
* spot/twaalgos/dot.cc: Implement the 'x' option and refactor the code
a bit to limit duplication.
* tests/core/dot2tex.test: New test case (requires dot2tex).
* tests/Makefile.am: Add dot2tex.test.
* tests/core/alternating.test, tests/core/readsave.test,
tests/python/automata-io.ipynb: Adjust expected output.
* NEWS, doc/org/oaut.org: Mention the new option.

tests/python/automata-io.ipynb

@@ -88,9 +88,9 @@
         "  size=\"10.2,5\" edge[arrowhead=vee, arrowsize=.7]\n",
         "  I [label=\"\", style=invis, width=0]\n",
         "  I -> 1\n",
-        "  0 [label=\"0\", peripheries=2]\n",
+        "  0 [label=<0>, peripheries=2]\n",
         "  0 -> 0 [label=<1>]\n",
-        "  1 [label=\"1\"]\n",
+        "  1 [label=<1>]\n",
         "  1 -> 0 [label=<b>]\n",
         "  1 -> 1 [label=<a &amp; !b>]\n",
         "}\n",
@@ -862,4 +862,4 @@
    "metadata": {}
   }
  ]
-}
+}