Drop-send multiple files - ask for all files at once

[Simon-Laux]

A workaround could be to show only one dialog that lists the files and asks if you are sure to send them all.

Originally posted by @Simon-Laux in #1552 (comment)

I believe this behavior would be better than the current showing of multiple dialogs.
Alone the reason that the backdrop adds up to much in the current version with multiple dialogs adds up waaaay too much.