* lisp/textmodes/page-ext.el: Misc cleanup, add RET binding

* lisp/textmodes/page-ext.el: Misc cleanup, add RET binding

Use lexical-binding.
Remove redundant :group args.
(pages-directory-mode-map): Bind RET.
(pages-buffer, pages-pos-list): Define as buffer-local.
(pages-directory-map): Define as alias *before* the defvar.
Mark as obsolete.
(pages-buffer-original-position, pages-buffer-original-page):
Move declaration to before the first use.
(pages-directory): Remove unused var `linenum`.
(pages-directory-goto): Add optional `event` arg and make it work when
bound to mouse events.
(pages-directory-goto-with-mouse): Make it an obsolete alias.