[Folder View] Use a copy of QMimeData in signal handler

By the time we leave the method, the QMimeData is deleted by the DragArea's drop event being destroyed leading to a crash.

CHANGELOG: Fixed crash when dropping files on desktop with KDeclarative from KDE Frameworks 5.35

BUG: 380806
FIXED-IN: 5.10.1

Differential Revision: https://phabricator.kde.org/D6088