This merges the public `uiItemS` and `uiItemS_ex` functions into an
object oriented API (`uiLayout::separator`), matching the python API.
This reduces the difference between the C++ API with the python version,
its also helps while converting code from python to C++ code
(or vice-versa), making it almost seamless.
Part of: #117604
Pull Request: https://projects.blender.org/blender/blender/pulls/138826