* lisp/emacs-lisp/package.el: Implement displaying obsolete packages

* lisp/emacs-lisp/package.el: Implement displaying obsolete packages

(package-menu--hide-obsolete): New variable.
(package--remove-hidden): Use it.
(package-menu-hide-obsolete): New interactive function to toggle
the variable.
(package--quick-help-keys): Document it.
(package-menu-async): Add :version tag.
(package-menu-mode-map): Bind package-menu-hide-obsolete.
(package-desc-status): Indicate non-installed obsolete packages as
avail-obso.
(package-menu-mark-install): Allow installation of avail-obso.
(package-menu--status-predicate): Sort avail-obso with available.