* wl-e21.el (wl-e21-setup-draft-toolbar): Don't refer to `wl-use-toolbar' nor

* wl-e21.el (wl-e21-setup-draft-toolbar): Don't refer to `wl-use-toolbar' nor
 `display-graphic-p'.
(wl-e21-setup-message-toolbar): Ditto.
(wl-e21-setup-summary-toolbar): Ditto.
(wl-e21-setup-folder-toolbar): Ditto.

(wl-biff-init-icons): Rewrite using `wl-e21-display-image-p'.
(wl-plugged-init-icons): Ditto.

(wl-folder-init-icons): Use `wl-e21-display-image-p' instead of
 `display-graphic-p'.
(wl-plugged-set-folder-icon): Ditto.
(wl-highlight-plugged-current-line): Ditto.
(wl-highlight-folder-current-line): Ditto.
(wl-e21-highlight-folder-group-line): Ditto.
(wl-e21-setup-toolbar): Ditto.
(wl-e21-display-image-p): New macro.

* wl-demo.el (wl-demo): Simplified.
(wl-demo-image-type-alist): Use `image-type-available-p' for checking whether
 the image type `xbm' is available.