Create copyOnAdd props

[David-Desmaisons]

Proposed props:

copyOnAdd
Type: Function
Required: false
Default: (original) => { return original;}

Function called on the target component on drag end to copy an element when it is added by dropping. The unique argument is the viewModel element to be copied and the returned value is the element to be added.
Note that it differs from the clone props that is called on the source component and the drag start.

[David-Desmaisons]

From issue #586