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.