* wl-highlight.el (wl-highlight-summary-prefetch-face): New face.

* wl-highlight.el (wl-highlight-summary-prefetch-face): New face.
(wl-highlight-summary-line-string): Highlight for "i" mark.
(wl-highlight-summary-current-line): Ditto.

* wl-thread.el (wl-thread-print-destination-region): Avoid error when
no destination.

* wl-summary.el (wl-summary-prefetch-msg): If cache file already
exists,	just go ahead.
(wl-summary-prefetch-region): Rewrite.
(wl-summary-prefetch): Ditto.
(wl-summary-mark-action-list): Define mark "i" and prefetch action.
(wl-summary-exec-action-prefetch): New function.
(wl-summary-target-mark-prefetch): Rewrite.