[TASK] Move Backend AJAX Calls to index.php

The typo3/ajax.php entrypoint is removed in favor of
checking for ajaxID in all backend requests inside the Backend
Application.

The request object from the server is now enhanced with the
information inside the application which type is now called.

In this course, the GeneralUtility::isInternalRequestType()
is extended to return ture if no request type constant is set
due to early calls to GeneralUtility methods when building the
ServerRequest object with a Uri object.

Resolves: #68141
Releases: master
Change-Id: I27f2e4fce2007981f323d0607c2dfb0b3a17d020
Reviewed-on: http://review.typo3.org/41247


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>