[TASK] Compatibility with TYPO3 6.2

Fix various issues to ensure compatibility with TYPO3 CMS 6.2:

* some Core JS resources have been moved around
* some Core styles have changed
* Scheduler tasks have a new field, task group, which must be set properly
* a work around to an ExtJS display bug needs improvement

Resolves: #58020
Releases: 2.3
Change-Id: Id4fda80a6e5f81c52d2b8e44c261350d966fe32e
Reviewed-on: https://review.typo3.org/29692
Reviewed-by: Francois Suter
Tested-by: Francois Suter