* lisp/mail/footnote.el: Add TEXT and POINTERS together

* lisp/mail/footnote.el: Add TEXT and POINTERS together

Rather than adding POINTERS and TEXT separately to footnote--markers-alist,
add them together, so we don't need footnote--first-text-marker because
the TEXT part is never nil.

(footnote--insert-numbered-footnote): Return marker.
(footnote--insert-text-marker, footnote--insert-pointer-marker):
Delete functions.
(footnote--insert-markers): New function to replace them.
(footnote--insert-footnote): Adjust accordingly.
Simplify pointless `unless`.
(footnote--first-text-marker): Remove.  Replace all calls by
(cadr (car footnote--markers-alist)) or just footnote--markers-alist.