cex: color the counterexamples

Use colors to show the counterexamples and the derivations in color,
to highlight their structure.  Align the outputs, and add i18n
support. Reduce width by using a one-space separator instead of
two-space.

From

    Example  A  •  B  C
    First  derivation  s ::=[ a ::=[ A  • ]  x ::=[ B  C ] ]
    Second derivation  s ::=[ y ::=[ A  •  B ]  c ::=[ C ] ]

to

    Example              A • B C
    First derivation     s ::=[ a ::=[ A • ] x ::=[ B C ] ]
    Example              A • B C
    Second derivation    s ::=[ y ::=[ A • B ] c ::=[ C ] ]

with colors.

* data/bison-default.css (cex-dot, cex-0, cex-1, cex-2, cex-3, cex-4)
(cex-5, cex-6, cex-7, cex-step, cex-leaf): New.
* src/derivation.c (derivation_print_styled_impl): New.
(derivation_print, derivation_print_leaves): Use it.
* src/counterexample.c: Reformat the output.
* tests/counterexample.at: Adjust.

src/derivation.h

@@ -20,6 +20,7 @@
 #ifndef DERIVATION_H
 # define DERIVATION_H
 
+# include <gl_linked_list.h>
 # include <gl_xlist.h>
 
 # include "gram.h"