Feature: Allow canvas to programmatically allow/reject incoming and outgoing nodes

[prevwong]

Canvas needs to be able to check whether the incoming/outgoing component is allowed.

Current proposal:

<Canvas
  incoming={(node: Node, children: Array<Node>) => {
    // Logic happens here
    if (node.name === "Button") {
      const count = children.filter(child => child.name === "Button").length;
      if (count > 3) { // too many buttons in container already
        return false; 
      }
    }
    return true;
  }}
  outgoing={(node: Node, children: Array<Node>) => {
    // Logic happens here
    if (node.name === "Button") {
      const count = children.filter(child => child.name === "Button").length;
      if (count <= 1) { // too few buttons, so prevent current button from being moved out
        return false; 
      }
    }
    return true;
  }}
>
...
</Canvas>