Set a better defaultDropAction for dragging

Summary: Because we use KIO::DropJob to determine the dropAction, so changing this one does not change anything else except the default cursor from the copy icon to the closed-hand one.

Test Plan: Drag files without holding any modifier key and see the cursor.

Reviewers: #dolphin, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D28370