onCanvasDrop: Support id from data argument

[NoyTse]

When using external state and drag&drop, it may be useful to store the new item in DB / Redux when the onCanvasDrop event is fired, but still wrap the chart with component with inner-state, and use the default "action handlers" from the libary to maintain the state.

So, with this patch we can choose the id in

onDragStart={(ev) => { 
    ev.dataTransfer.setData(
        REACT_FLOW_CHART, 
        JSON.stringify({ id: chosenId, ...newNode) }
    ) 
}}

hope you found this helpfully,
Noy