Minor Tidy up of RequestHandlerComponent around AJAX

NOTE: This comment has been updated from original proposal.

Tries to make it more clear how AJAX is handled by doing as it consistently as possible with other types.

* Deprecates RequestHandler::$ajaxLayout, and adds AjaxView class to handle Ajax specific view settings, which is really only 'layout' but still make for cleaner more customizable code.
* Adds MIME mappping 'ajax' => 'text/html' in Response in order to remove some AJAX special casing in RequestHandler.
* Some test changed accordingly. Deleted one test.
* Comments.
* Also one liner fix to treat 'htm', and 'html' that same everywhere in there.