[TASK] Followup improve "Overwrite files" message

Make it possible to use exiting file when uploading files directly in
content element. Finetuned the handling of existing files + loading
bar behavior. And make the modal behave correct with multiple files.

Resolves: #68235
Releases: master
Change-Id: I2b307dd0e90e38ad58770b3fa28425c747b5c05b
Reviewed-on: http://review.typo3.org/41582


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>