[NME] Prevent alt+click shortcut to drop elbow on complex node connections

[PatrickRyanMS]

Elbows cannot handle complex node connections like those between ImageAssetBlock and TetxtureBlock

If the user tries to drop an elbow on a connection between complex nodes, the elbow will place with no connections. Going forward any elbow dropped with the shortcut on any connection will fail and only place an unconnected elbow.

Repro

• Bug repro on NME Sample:
• alt + click on the blue connection between ImageAssetBlock and TextureBlock
• note that the elbow is not connected to anything
• next try to alt + click on any other connection, either the Vector2 or Float connections. Note that the elbow also drops unconnected and now the shortcut does not work anymore.

It would be best if we disallow the shortcut to create an elbow if the user is hovering a connection that is not allowed by the elbow. I assume that eliminating this as an option to create an elbow will avoid the state that breaks the shortcut after trying to add an elbow on a complex node connection.