Node graph QoL improvements

[adamgerhant]

• Fixes inserting a layer to the top of a stack with multiple outputs, now transferring all outputs to the new top layer
• Fixes layer positioning logic which caused too much space to be created after inserting at the top of a stack
• Fixes shaking a node to disconnect it when it's connected to an import

Future improvements (deferred):

• Fix incorrect node duplication
• Fix export position when disconnecting layer