Dont special case AJAX in RequestHandlerComponent (much).

Adds a AjaxView view class and an additional default mapping from 'ajax' mim type alias to this class.
This means $RequestHandlerComponent::_defaultConfig['ajaxLayout'] is deprecated. Also added a MIME map for ajax in Response.php.
Updates to related tests.
Minor change to handle htm,html the same in RequestHandler _ext mangling.
Comments.
Comments only.
Move responsibility for setting default view vars from RequestHandler to XML,JSON views. CS fixes.
Added three new test cases for RequestHandlerComponent rel View classes.
Minor doc fixes.
Added test for correct mangling of response when route extension added but no view class set.
Merge pull request #2932 from sam-at-github/requesthandlercomponent

Minor Tidy up of RequestHandlerComponent around AJAX