Warn or handle duplicate node labels

[jdanish]

Added based on PR #39

The ideal functionality is to receive a warning (popup is fine) that allows the user to either continue, or edit the original node. Less ideal is to just have the warning and then they can opt to cancel.

Edited to add ideal functionality.

[jdanish]

Note: if you start to create one then realize you made a mistake, cancel does not delete the duplicate...

[jdanish]

@benloh can we add this to the "during December" list? Assuming it isn't a major hurdle.

[benloh]

I'll add it to the list

[benloh]

See also #38

[benloh]

TO DO

• "Duplicate" warning only shows on NCNode, not on NCSearch.
• Show duplicate warning on NCNode also, not just AutoComplete?
• Show ID number in AutoComplete?
• Show ID number in Edge Source/Target?

[benloh]

Implemented with #113