port: add the dragged port as the drag icon
This makes it the port label follows your cursor around instead of the generic text document icon that doesn't make a lot of sense here.
I don't know if there's a better thing to put there, maybe something with a jack icon, but this seems useful to remember what you're dragging. Mostly this is because I found the default text document drag to be jarring in this context.
Edited by Carlos Martín Nieto