[BUGFIX] Improve confirmation dialog in drag'n'drop fileupload

This patch introduces the following changes to the drag'n'drop
fileuploader:

* Only ask for confirmation to overwrite files, if at least
one of the uploaded files will overwrite an existing file on
the server
* Display the files which will be overwritten in the confirmation
dialog
* If the user selects "Cancel", cancel the whole action, and
don't upload any files.
* If no files will be overwritten, just upload them without
disturbing the user with a confirmation message.

Resolves: #53776
Releases: 6.2
Change-Id: I415c2d31b5bf1248f8a865d225a81ddd96e47d78
Reviewed-on: https://review.typo3.org/25523
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters