[5.0][Events] Event class for com_ajax requests

[Fedik]

Summary of Changes

Event class for com_ajax requests, for 'onAjax...' events.
The event allows not only apped the result, also allows to set a whole result, wich was a missing feature.

Testing Instructions

Install sample data, or use Debug plugin and view debug history.

Actual result BEFORE applying this Pull Request

Works

Expected result AFTER applying this Pull Request

Works

Link to documentations

Please select:

• Documentation link for docs.joomla.org: https://docs.joomla.org/Using_Joomla_Ajax_Interface#Plugin_Response
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org: New Event classes Manual#177
• No documentation changes for manual.joomla.org needed

[HLeithner]

Merging this as with code review and based on the other similar PRs, before beta1 to get enough feedback for final release.