* lisp/mail/footnote.el: Consolidate the two marker-alists

* lisp/mail/footnote.el: Consolidate the two marker-alists

Consolidate footnote-text-marker-alist and footnote-pointer-marker-alist
into a single footnote--markers-alist.

(footnote--markers-alist): New var.
(footnote-text-marker-alist, footnote-pointer-marker-alist): Delete vars.
(footnote--refresh-footnotes, footnote--text-under-cursor)
(footnote--calc-fn-alignment-column, footnote-add-footnote)
(footnote-goto-footnote, footnote-back-to-message): Adjust accordingly.
(footnote--make-hole, footnote-delete-footnote)
(footnote-renumber-footnotes): Simplify accordingly.
(footnote-cycle-style): Indicate style name in echo area.
(footnote--renumber): Take a single `alist-elem` arg instead of
`pointer-alist` and `text-alist`.
(footnote--insert-text-marker, footnote--insert-pointer-marker):
Add to footnote--markers-alist instead.
(footnote--first-text-marker): New function.
(footnote--get-area-point-min): Use it.
footnote--goto-first): New function.
(footnote--insert-footnote): Use it.
(footnote-style-number): Use defvar-local.