[TASK] Consolidate JSimgCode in TSFE into additionalJavaScript

In order to streamline the javascript variables in
frontend rendering the functionality for using
JSimagecode (used by GMENU) should add its
code to the generic $additionalJavaScript and
not the hardly used "public $JSImgCode".

Releases: 6.3
Resolves: #60523
Change-Id: Iec3b5623348ce448c1827e30a831acb3fb82a415
Reviewed-on: http://review.typo3.org/31758
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>