Get edge's label from editEdge event

[sawatsky]

We should be able to get the label information for edges in the editEdge event.

Currently, we can get label information for nodes from editNode, so it makes sense to be able to get the label from editEdge as well.

This will require adding another event - or adjusting the current events - for editing edges, so that users aren't required to arbitrarily move an edge's link to trigger the editEdge event. Users should be able to edit an edge's label without any other adjustments.

For current workarounds: #2068

[mojoaxel]

@eymiha are you sure this is fixed with #2329 ?

[eymiha]

In examples/network/other/manipulationEditEdgeNoDrag.html you can see that now, when editing a labeled edge, the current value is being pulled from the passed data. Unless I really mucked something up it should be working because the label data is loaded in the ManipulationSystem#performEditEdge function.

[mojoaxel]

@eymiha Yes looks good.
@sawatsky Feel free to reopen if this is still an issue.