Correct the accept flag of the event object on DragMove

Summary:

• m_enabled or m_temporaryInhibition could change while DragMove, so we should place setAccepted() on top of the function body. Otherwise, the accept flag of event could be wrong in the case m_enabled or m_temporaryInhibition changes while DragMove.
• Don't call setAccepted(false) wrongly if event->pos() == m_oldDragMovePo

BUG: 396011

Test Plan:

• Drag a file from Dolphin -> Desktop (Desktop containment layout) -> too difficult -> fixed
• Drag an icon on Desktop (Folder view layout) from one place to another one -> too difficult -> fixed
• Add widgets to the Plasma panel -> too difficult -> fixed
• Drag a file from Dolphin -> Desktop (Folder view layout) -> still easy

Reviewers: mart, broulik, Plasma, hein, bruns

Reviewed By: mart, Plasma

Subscribers: fvogt, aacid, bruns, dkorth, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16643