Fix undo for edges

[moerketh]

Summary:
This change should fix the undo of edges in the model. Both when deleting a node with connected edges and a single edge.

Here is the undo behavior before this change (on the public website):

orig_small.mp4

Here is the undo behavior after this change (locally hosted):

fixed_small.mp4

Description for the changelog:
Register Edges in the x6 model instead of registering Edges as Nodes.

Other info:
Closes #428

Line 14 might need a similar change, but I am not familiar with the compatibility concerns.

[jgadsden]

many thanks @moerketh , good to have such an elegant bug fix