Define the `hideDropzone` event listener via assignment to keep the
binding of `this` to the DragUploader class.

Resolves: #90792
Releases: master, 9.5
Change-Id: I21c52da477349e77a52f2c6701fd75abc3b5b074
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63805


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>