calendar.el: Add new faces, and day-header-array

calendar.el: Add new faces, and day-header-array

* lisp/calendar/calendar.el (calendar-weekday-header)
(calendar-weekend-header, calendar-month-header): New faces.
(calendar-day-header-construct): New function.
(calendar-day-header-width): Also :set calendar-day-header-array.
(calendar-american-month-header, calendar-european-month-header)
(calendar-iso-month-header): Use calendar- faces.
(calendar-generate-month):
Use calendar-day-header-array for day headers; apply faces to them.
(calendar-mode): Check calendar-font-lock-keywords non-nil.
(calendar-abbrev-construct): Add optional maxlen argument.
(calendar-day-name-array): Doc fix.
(calendar-day-name-array, calendar-abbrev-length)
(calendar-day-abbrev-array):
Also :set calendar-day-header-array, and maybe redraw.
(calendar-day-header-array): New option.
(calendar-font-lock-keywords): Use calendar-day-header-array,
and calendar- faces.  Make obsolete.
(calendar-day-name): Add option to use header array.

* etc/NEWS: Mention this.

Fixes: debbugs:15007