* Changed action=revert to use a subclass of Action

* Changed action=revert to use a subclass of Action
* Added WikiPage::getActionOverrides() to be able to execute different actions depending on the namespace (obviously needed for action=revert). This is only used when the value of $wgActions for the corresponding action is true; so extension can still override this.
* Added Action::getDescription() to ease the change of the page header and the <title> element