[BUGFIX] Toggle import button in Element Browser

The "Import selection" button in the Element Browser is now disabled by
default, as no selection is done yet. (De)selecting items toggles the
import button accordingly.

Resolves: #90869
Releases: master
Change-Id: I55e30abe83bf189c2825c30551b357cbd0174f44
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63992


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>