WICKET-5199 javadoc
Use constants
WICKET-5196 Make AjaxFormValidatingBehavior non-static
WICKET-5196 Make AjaxFormValidatingBehavior non-static
Remove a space character. No functional changes
Non functional changes:

- fix a typo in javadoc
- diamond operator
- use Character instead of String in concatenation
WICKET-5196 Improve Javadoc