* wl-vars.el (wl-summary-mark-action-list): Moved from wl-summary.el

* wl-vars.el (wl-summary-mark-action-list): Moved from wl-summary.el
and define using defcustom;
Define 4th element as a face.

* wl-summary.el (wl-summary-incorporate): Use
`wl-summary-prefetch-region-no-mark' instead of
`wl-summary-prefetch-region'.
(wl-summary-prefetch-region-no-mark): Revival of old
`wl-summary-prefetch-region'.
(wl-summary-mark-action-list): Moved to wl-vars.el

* wl-highlight.el (wl-highlight-summary-current-line): Decide
face according to the `wl-summary-mark-action-list'.
(wl-highlight-summary-current-line): Ditto;
Removed destination highlighting.