(font-lock-removed-keywords-alist): New variable.
(font-lock-add-keywords): Updates `font-lock-removed-keywords-alist'.
Empty `font-lock-keywords-alist' when `append' is `set' to avoid
- growing datastructures.
+ growing data structures.
(font-lock-set-defaults): Removes keywords stored in
`font-lock-removed-keywords-alist' after local keywords added.