WICKET-5564 AjaxRequestTarget.focusComponent() does not work when two Ajax responses arrive next to each other

Minor simplifications:
- use local variable for namespaced lookups
- simplify checks for truty values
- fix typos

(cherry picked from commit 37a8413dead0a344f7fbb03bd6c2b83cf84dce9a)
WICKET-5564 AjaxRequestTarget.focusComponent() does not work when two Ajax responses arrive next to each other

Focus the last component in the current event loop.
The timeout has been done with https://github.com/apache/wicket/commit/a4d7cea1c50c6640e124518af93afa2e14ca3220 at Jun 4 2007 for Internet Explorer. IE8 has been released Mar 19 2009 (http://en.wikipedia.org/wiki/Internet_Explorer_8). So the fix is for IE 6/7.
Since there is no info how to test whether the fix is still needed for IE8+ we will revert the fix and wait for feedback.

(cherry picked from commit 6d55c95a76a255578ed8158f9737cd3263dce709)