Add "clear selected posts" to the moderation toolbox

[Colonel_mortis]

Currently, if you select multiple posts with the checkboxes, the only ways to uncheck them are to perform an action on them, or to manually go back and uncheck them. If you decide that actually you don't want to perform any action on them, it can be quite time consuming going back through all the pages where you had selected posts/comments and unchecking them all. It would be helpful, and not very complicated, to add one more item to the moderation toolbox to "clear selected posts".