Dragging constraints for draggable items

[ishubin]

For free draggable items we should be able to specify the dragging constraints with element type. User has to have the possibility of restricting the movement of the items when it is dragged by another item or a group of items.

Things to consider:

• Either only restrict the movement by the rectangle (the shape does not matter)
• Restrict the movement by the shape outline. This one is a lot more challenging, since we would need to be able to tell whether the item is inside of the shape or not.