Re-implement Edge Delete button

[benloh]

In the revamp of NCEdge.jsx (from EdgeEditor.jsx), we did not implement the "DELETE" button. This adds it back in.

The new "DELETE" button is only visible in EDIT mode. I figure hiding the button behind edit makes it less likely to end up with a careless deletion.

NOTE: The "Cite Edge" button has not been implemented either.

To Test

1. Open a Graph
2. Click on a Node with an edge
3. Click on the "EDGES" tab
4. Select an edge
5. Click "EDIT" to edit the edge
6. Click "DELETE" to remove the edge
7. The edge should immediately be removed, and you will automatically exit EDIT mode.
8. You should be able to continue working, selecting other nodes, selecting ATTRIBUTES, working with edges etc.