WICKET-5650 Make is possible to position the choice label before/after/around the choice

(cherry picked from commit f9668b5ea6d4dfc196409dff588a06dd5b9ea571)
WICKET-5651 Add TagTester#getChild(String tagName) method

(cherry picked from commit 5d47267e94ba2eae0d188132774cc2dc61b03f68)