Magnify receiving node ports when edge dragged over

[bergie]

When user is creating a new edge, we should magnify the ports of a node (similarly to how they're shown in the node contextmenu) when the edge is dragged over a node.

They should be returned when user drags out of the node.

Incompatible ports should be greyed out, leaving only ports of:

1. same type as the port being dragged from
2. all type
3. bang type

visualized. See meemoo/dataflow#63 and noflo/dataflow-noflo#5 for the way we did this originally.

[forresto]

Need some design on this.

If they are going to expand when the pointer is to the left of the node, there needs to be a transparent element there to get the pointerenter. This would be applied only when the preview edge is present.

[forresto]

Also, number -> string?