qt: set fill mode to preserve aspect crop in `DragItem`

DragItem expects that the image fills the area denoted for the image.

I did not realize this before because I used tracks for testing the drag item with the new round image, where they are usually square so the default PreserveAspectFit did not make it obvious.

One might ask why not set Image.PreserveAspectCrop the default if it is used more than Image.PreserveAspectFit. Even though we don't need a clip node (as long as there is radius at the moment), unnecessary Image.PreserveAspectCrop still causes using a more computationally expensive shader. We should not use Image.PreserveAspectCrop unnecessarily.

Request review @chub.