Get edge's label from editEdge event

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 (closed)