[CLEANUP] FormEngine suggest: Split up AJAX method

This moves parts of the very complex AJAX receiver method to their own
methods, making the original method a lot shorter and easier to
understand.

Resolves: #67593
Releases: master
Change-Id: I2422313e988195c3c12ac9133542f1f6183cc5a4
Reviewed-on: http://review.typo3.org/40422


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>