New implementations of highlighting Folder.

New implementations of highlighting Folder.

* wl/wl-xmas.el (wl-xmas-highlight-folder-group-line): New function.
(wl-highlight-folder-current-line): Use it; new implementation.
(wl-xmas-setup-draft-toolbar, wl-xmas-setup-message-toolbar,
wl-xmas-setup-summary-toolbar, wl-xmas-setup-folder-toolbar): Use `defsubst'
instead of `defun'.

* wl/wl-vars.el (wl-highlight-folder-by-numbers): Renamed from
`wl-highlight-group-folder-by-numbers'; made it can also be a number.  See
info for more details.

* wl/wl-summary.el: Bind `wl-xmas-setup-summary' when XEmacs is not running.

* wl/wl-nemacs.el (wl-xmas-setup-*, wl-delete-all-overlays): No need to bind
them.

* wl/wl-mule.el (wl-xmas-setup-*): No need to bind them.
(wl-highlight-folder-current-line): New implementation.

* wl/wl-highlight.el (wl-highlight-folder-group-line): New implementation.
(wl-delete-all-overlays): Rewrite as a marco.
(TopLevel): Require `wl-e21' when Emacs 21 is running.

* wl/wl-folder.el: Bind `wl-xmas-setup-folder' when XEmacs is not running.
(wl-folder-*-glyph): No need to bind them.

* wl/wl-e21.el (wl-e21-highlight-folder-group-line): Renamed from
`wl-e21-highlight-folder-group-icon'; rewrite.
(wl-e21-setup-draft-toolbar, wl-e21-setup-message-toolbar): Use `defsubst'
instead of `defun'.
(wl-folder-mode-map): Bind it when compiling.

* doc/{wl-ja.texi, wl.texi}: Replace `wl-highlight-group-folder-by-numbers'
with `wl-highlight-folder-by-numbers'; add description about
`wl-highlight-folder-by-numbers'; update for Emacs 21.

* WL-ELS (WL-MODULES): Add `wl-e21'.