Dispatchers should accept Boxes

[andrewkaufman]

We want to make Dispatcher::dispatch() accept Boxes, and dispatch any internal ExecutableNodes whose outputs have been promoted to the Box. That'll allow users to create custom networks and then present them as a new type of node.

We also need to change ExecutableNode::acceptsInput() to accept inputs from Boxes, in the same way as we do in other acceptsInput() implementations (ShaderAssignment::affectsInput() is a good example).