8b610723b6
With 9d0907560a, button tooltips would now often display the label even though
it's already visible in the button itself.
This fix brings back the old logic for displaying the label, but it's still
possible to override the label via the callback introduced in the earlier
commit.
Comments are updated/added here to make the behavior more clear.
Pull Request: https://projects.blender.org/blender/blender/pulls/110469