dot: add an option to output id= attributes

This will be handy latter to develop widgets with interactive
highlighting of automata.

* spot/twaalgos/dot.cc: Implement it.
* bin/common_aoutput.cc, NEWS, doc/org/oaut.org,
doc/org/spot.css: Document it.
* tests/core/alternating.test, tests/core/readsave.test,
tests/core/sccdot.test: Test it.

tests/core/sccdot.test

@@ -1,7 +1,7 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2015, 2017, 2018, 2020 Laboratoire de Recherche et
-# Développement de l'Epita
+# Copyright (C) 2015, 2017, 2018, 2020, 2021 Laboratoire de Recherche
+# et Développement de l'Epita
 #
 # This file is part of Spot, a model checking library.
 #
@@ -68,7 +68,7 @@ State: 10
 --END--
 EOF
 
-run 0 autfilt --dot=as in.hoa > out.dot
+run 0 autfilt --dot=asi in.hoa > out.dot
 
 # The important stuff is the color=xxx lines
 cat <<EOF >expected
@@ -77,79 +77,88 @@ digraph "" {
   label="(Inf(0)&Inf(1)) & Fin(2)\n[Streett-like 3]"
   labelloc="t"
   node [shape="ellipse",width="0.5",height="0.5"]
+  node [id="S\N"]
   I [label="", style=invis, width=0]
   I -> 1
   subgraph cluster_0 {
   color=grey
+  id="SCC0"
   label=""
   5 [label="5"]
   6 [label="6"]
   }
   subgraph cluster_1 {
   color=grey
+  id="SCC1"
   label=""
   0 [label="0"]
   }
   subgraph cluster_2 {
   color=green
+  id="SCC2"
   label=""
   9 [label="9"]
   10 [label="10"]
   }
   subgraph cluster_3 {
   color=green
+  id="SCC3"
   label=""
   8 [label="8"]
   }
   subgraph cluster_4 {
   color=green
+  id="SCC4"
   label=""
   7 [label="7"]
   }
   subgraph cluster_5 {
   color=black
+  id="SCC5"
   label=""
   2 [label="2"]
   }
   subgraph cluster_6 {
   color=red
+  id="SCC6"
   label=""
   4 [label="4"]
   }
   subgraph cluster_7 {
   color=green
+  id="SCC7"
   label=""
   1 [label="1"]
   3 [label="3"]
   }
-  0 -> 0 [label="a & b\n{0,1,2}"]
-  0 -> 0 [label="!a & !b\n{2}"]
-  0 -> 5 [label="a\n{2}"]
-  1 -> 4 [label="b"]
-  1 -> 3 [label="a & !b"]
-  2 -> 0 [label="a"]
-  2 -> 7 [label="b"]
-  3 -> 1 [label="a & b\n{0,1}"]
-  4 -> 4 [label="!b\n{1,2}"]
-  4 -> 2 [label="b"]
-  5 -> 6 [label="1"]
-  6 -> 5 [label="1"]
-  7 -> 7 [label="!a & b\n{0,1}"]
-  7 -> 7 [label="a & b\n{0,2}"]
-  7 -> 8 [label="1"]
-  8 -> 8 [label="!a & b\n{0,2}"]
-  8 -> 8 [label="a & b\n{0,1}"]
-  8 -> 9 [label="1"]
-  9 -> 9 [label="!a & b\n{0,2}"]
-  9 -> 10 [label="a & b\n{0,1}"]
-  10 -> 9 [label="!a & b\n{0,1}"]
-  10 -> 10 [label="a & b\n{0,2}"]
+  0 -> 0 [label="a & b\n{0,1,2}", id="E1", tooltip="\\\\E\n#1"]
+  0 -> 0 [label="!a & !b\n{2}", id="E2", tooltip="\\\\E\n#2"]
+  0 -> 5 [label="a\n{2}", id="E3", tooltip="\\\\E\n#3"]
+  1 -> 4 [label="b", id="E4", tooltip="\\\\E\n#4"]
+  1 -> 3 [label="a & !b", id="E5", tooltip="\\\\E\n#5"]
+  2 -> 0 [label="a", id="E8", tooltip="\\\\E\n#8"]
+  2 -> 7 [label="b", id="E9", tooltip="\\\\E\n#9"]
+  3 -> 1 [label="a & b\n{0,1}", id="E10", tooltip="\\\\E\n#10"]
+  4 -> 4 [label="!b\n{1,2}", id="E6", tooltip="\\\\E\n#6"]
+  4 -> 2 [label="b", id="E7", tooltip="\\\\E\n#7"]
+  5 -> 6 [label="1", id="E11", tooltip="\\\\E\n#11"]
+  6 -> 5 [label="1", id="E12", tooltip="\\\\E\n#12"]
+  7 -> 7 [label="!a & b\n{0,1}", id="E13", tooltip="\\\\E\n#13"]
+  7 -> 7 [label="a & b\n{0,2}", id="E14", tooltip="\\\\E\n#14"]
+  7 -> 8 [label="1", id="E15", tooltip="\\\\E\n#15"]
+  8 -> 8 [label="!a & b\n{0,2}", id="E16", tooltip="\\\\E\n#16"]
+  8 -> 8 [label="a & b\n{0,1}", id="E17", tooltip="\\\\E\n#17"]
+  8 -> 9 [label="1", id="E18", tooltip="\\\\E\n#18"]
+  9 -> 9 [label="!a & b\n{0,2}", id="E19", tooltip="\\\\E\n#19"]
+  9 -> 10 [label="a & b\n{0,1}", id="E20", tooltip="\\\\E\n#20"]
+  10 -> 9 [label="!a & b\n{0,1}", id="E21", tooltip="\\\\E\n#21"]
+  10 -> 10 [label="a & b\n{0,2}", id="E22", tooltip="\\\\E\n#22"]
 }
 EOF
 
 diff out.dot expected
 
-# While we are here, make sure scc_filter remove those grey SCCs.
+# While we are here, make sure scc_filter removes those grey SCCs.
 autfilt --small -x simul=0 in.hoa -H > out.hoa
 cat >expected.hoa <<EOF
 HOA: v1