Drop edge to canvas to pop-up new node dialog

[bergie]

When user drags a wire from a port, and drops on the canvas, we should allow them to add a new node there.

This should bring up a visual "placeholder node". If user does some other interaction next, it should go away. If user clicks it, we should show a NoFlo UI library list filtered down to components that are compatible with the port type.

So, essentially we need a way to supply a "newNode" callback to the-graph-editor that gets triggered with the port info and coordinates when the placeholder is clicked.

[forresto]

Unreal Engine 4 has some of the interactions we have been talking about: drag to blank -> filtered library search inplace: http://youtu.be/8ew12FLgaMs