Add support for different plugin groups to com_ajax

Currently, `com_ajax` is hardcoded to load the "Ajax" plugin group.
This PR proposes to add support for a `group` URL parameter which tells com_ajax to import a custom plugin group. If omitted it will fall back to the ajax group.

The reason is that we may have for example a content plugin which uses Ajax functionality. With the current implementation it would neeed two plugins, one in the content group for the markup/whatever and a second one in the ajax group to process the AJAX requests. That doesn't make much sense.

#### Testing
* You can use the example plugin https://github.com/Joomla-Ajax-Interface/Ajax-Latest-Articles to see that nothing breaks when using `index.php?option=com_ajax&plugin=latestarticles&format=debug` (should give you a list of articles).
* To test the new group parameter you can add `&group=content` to the URL and see that the example plugin isn't triggered anymore (you get an empty array).
* Now you can copy the function `onAjaxLatestarticles` from the example plugin to a content plugin of your choice and it should work again.