WICKET-5197 Rename AjaxRequestAttributes#allowDefault to #preventDefault

related-to: https://github.com/apache/wicket/commit/66925ddf91ea931adf874439e24f3c68ddf81919
requested-by: http://markmail.org/message/7iep4uwijzcxkpr2
WICKET-5724 better comment and fix whitespaces
WICKET-5731 Using "Submit" button in Ajax DropDownChoice example leads to broken HTML page
WICKET-5197 Rename AjaxRequestAttributes#allowDefault to #preventDefault

(AJAX Guestbook example was broken)
WICKET-5731 Using "Submit" button in Ajax DropDownChoice example leads
to broken HTML page
WICKET-5730 Dequeue auto component can't resolve components if they are
nested in child markup