Tasos Sahanidis authored Jan 08, 2020 and Nate Graham committed Jan 08, 2020

Summary:
It is possible to begin a drag & drop sequence using the forward/backward mouse buttons while moving the cursor.
For example:

> QGraphicsSceneMouseEvent(GraphicsSceneMouseMove, buttons=ForwardButton|XButton2|ExtraButton2, pos=347,127, scenePos=347,127, screenPos=398,255)

This patch only allows drag & drop if the left mouse button is pressed.
Touch events should not be affected as they are handled separately.

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: ngraham

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D26507