Unify dragging images to scene view and as layer child in the timeline tree

Currently it is possible to drag an image to scene view to create a rect with new material based on that image as material. Similarly, it is possible to drag the an image under layer in the timeline tree to create a rect. The rect and material creation process in these two cases is entirely different, though, and results in different kind of material and even different size rect.

The behaviour of rect creation in these two cases should be unified.