Update dragCursor while dragging

Summary:
While dragging, the user could want to change the modifier, so we should call event->acceptProposedAction() to do update the dragCursor.

FIXED-IN: 20.04.0

Test Plan: While dragging files, change modifiers between Ctrl/Shift/Alt... and move the mouse at least 1px to see the cursor changes.

Reviewers: Dolphin, meven, ngraham, davidedmundson, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D28017