when two nodes clicked in existing graph, new deprel is created overwriting existing one

[jonorthwash]

When two nodes are clicked on the graph to create a deprel, a new deprel is created which overwrites any existing one. This results in data loss.

Perhaps it should just do nothing? It could either stay in "need to select 2nd node" mode, or return to "looking at the graph (and not editing deprels)" mode.

[nschneid]

Or it could request confirmation with a modal dialog ("Overwrite existing deprel?"), though that could get annoying.

Honestly the current behavior might be fine as long as it's visually salient that an old deprel is disappearing, and the undo button is available to recover if it was accidental.