Interact Nodes: Cleaning up logic in select-at method.
Interact Nodes: Implementing moving nodes.

 * Node#register-output is now symmetric with Input#register-input so
   that the source node is aware of the wire it's sending down.
 * Fixed bug in WireView#_make-wire where WireView.startpos and
   WireView.endpos are not kept in sync with the translations.