[TASK] Code cleanup FormEngine.js

The code of FormEngine.js is a bit cleaned up:
- Initialize variables explicitly
- Fix a typo: "undefinied" > "undefined"
- Chain event listeners

Change-Id: I3541a895622b109d1afdc805b1540f6ff1f44114
Resolves: #72498
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/45535


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>