[BUGFIX] Change path to label of preview pane in admin panel

the label is moved to a separate locallang file.
Hence the method getLabel needs to get the label from the new file.

Resolves: #84519
Releases: master
Change-Id: I16a961499ad7ac9a26d2cc92a7ea3ef80413fc15
Reviewed-on: https://review.typo3.org/56418


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Steffen Frese <steffenf14@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>