[!!!][FEATURE] Introduce PSR-7-based Routing for Backend AJAX Requests

The AjaxRequestHandler now first checks in the Router if an AJAX
route exists. A new flag "ajax" in the routing mechanism allows to call
ajax-based URLs which are then handed to the AJAX Request Handler.

All controllers now receive proper Request and Response objects.

All previous logic still works, but can slowly be migrated to the Routing
concept.

Resolves: #69916
Releases: master
Change-Id: I1e67d5a341a4dd2769247531246c9e1fad900c76
Reviewed-on: http://review.typo3.org/43365


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>