[REG 5.8 -> 5.9] Windows: Items start being dragged after single click

1. Open a form in Designer.
2. Click on one of the items in the Widgetbox.
   It will attach to your mouse pointer. You can now drag it around although you already released the mouse button.

Dragging should only happen while the mouse button is held down. In Qt 5.8, this used to work correctly.

Note: Unrelated to Designer, same happens with draggableicons.