[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