[TASK] Adjust loading animation of modal windows

Use TYPO3 spinner which is properly centered.

Releases: master, 8.7
Resolves: #83218
Change-Id: Ieec359ee0bc05ebb75724b963edaf1dc5bbab0a8
Reviewed-on: https://review.typo3.org/54929


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>