Factor out the magic numbers controlling the calendar layout.

Factor out the magic numbers controlling the calendar layout.
(calendar-month-digit-width, calendar-month-width)
(calendar-right-margin): New variables.
(calendar-recompute-layout-variables, calendar-set-layout-variable):
New functions.
(calendar-left-margin, calendar-intermonth-spacing)
(calendar-column-width, calendar-day-header-width)
(calendar-day-digit-width): New options.
(calendar-first-date-row): New constant.
(calendar-move-to-column, calendar-ensure-newline): New functions,
replacing calendar-insert-indented.
(calendar-insert-indented): Remove function.
(calendar-generate-month): Use calendar-move-to-column and
calendar-ensure-newline.  Use layout variables.
(calendar-generate, calendar-update-mode-line)
(calendar-font-lock-keywords): Use layout variables.
(calendar-column-to-month): New function.
(calendar-cursor-to-date): Use calendar-column-to-month.
Use layout variables.