Exclude build dependencies when counting inputs to new nested node

[glopesdev]

Since build dependencies always affect their target node and are not considered proper inputs to operators, they should be excluded when estimating number of inputs to a newly created nested node.

Fixes #1792

[PathogenDavid]

How difficult would it be to add a test for this? This fix seems like a good candidate for a test but I know testing stuff in the UI layer can be a pain

[glopesdev]

How difficult would it be to add a test for this? This fix seems like a good candidate for a test but I know testing stuff in the UI layer can be a pain

@PathogenDavid Thanks for the remainder, definitely important to keep doing this, especially for these edge cases, and especially for preparing the migration to a different UI layer.

It turned out to be a bit more difficult than expected, but not because of the UI. The main obstacle was I had to add testing infrastructure for branches, and also had to remember that the initial state of the workflow must be topologically sorted to ensure reversibility of undo/redo operations, so was good to make that explicit at least in a comment.

I think now TestWorkflow can be used to build pretty much any workflow programmatically so can almost be considered an API for a scripting editor or DSL.