[#32146] com_ajax: improvement, use controllers (new MVC), better error handling

changes:
 - `format` parameter now is required
 - own Exception handler that should fix previous problem with handling errors when "ajax" call,
 - own function for check "whether module available"
 - added `content` and `system` plugin groups

Links:
- old pull #2107
- Joomla! tracker [ [#32146] com_ajax: improvement, use controllers, better error handling ](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32146&start=0)