libtextstyle: Add support for true-color terminals.

libtextstyle: Add support for true-color terminals.

* gnulib-local/lib/term-ostream.oo.c (colormodel_t): Add cm_xtermrgb.
(rgb_to_color_xtermrgb): New function.
(attributes_t): Extend the color fields to 25 bits.
(struct term_ostream): New fields active_attr_color, active_attr_bgcolor.
(out_color_change, out_bgcolor_change, term_ostream::rgb_to_color): Handle
cm_xtermrgb.
(out_attr_change): Set also active_attr_color, active_attr_bgcolor.
(async_set_attributes_from_default): Fetch also active_attr_color,
active_attr_bgcolor.
(term_ostream_create): Accept all $TERM values that end in -direct.
* libtextstyle/doc/libtextstyle.texi (Terminal emulators): Document the
prerequisites for true-color support.
* libtextstyle/NEWS: Mention the change.