Be able to instantiate custom node by dragging-and-dropping file onto canvas

@alexmitchellmus asked on https://vuo.org/node/1056:

how do we make it so drag&dropping works with files? So that when I drop a .scl [file] onto the canvas the correct node is initiated.

Currently there's no public API for that. For this feature request, a node class would be able to specify the file extensions for which dragging-and-dropping a file onto the canvas should create an instance of the node class.