Give some kind of indication on navigator when a relevant modifier key is pressed

When dragging items around in navigator, you can use ALT key to adjust drop behavior.

In case of 2D items, it'll cause "anchor.fill: parent" to be inserted on drop.

In case of 3D nodes, it'll suppress adjusting the local transform properties of the dropped node that is normally done to keep the node visually in the same scene position.

Currently there is no indication that anything special is going to happen when ALT is pressed during the drag. It was suggested that mouse cursor should change somehow to indicate this, or maybe there's some better solution (or maybe it is not necessary to inform user at all). UX should determine what should happen in this case.