Built-in nodes already used the functionality to hide the input labels in a few
situations. However, this functionality was not exposed for node groups before.
It required some refactoring to get the semantics of the flag right. The tricky
aspect is that "Hide Label" would be an incorrect name, because the label is
still shown under various circumstances. Instead, the flag merely indicates that
drawing code may skip drawing the label.
This adds a new "Optional Label" input for node group inputs. Other names are
possible like "Requires Label" which would be the inverse.
Overall the implementation is pretty straight forward after
1f489ea31a,
469a70dba9 and
f79896f5b9.
Pull Request: https://projects.blender.org/blender/blender/pulls/146939
ef92735a95