This converts the public `uiLayoutSplit` function to an object oriented API (`uiLayout::split`), 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. `uiLayout::split` now returns an `uiLayout` reference instead of a pointer. New calls to this method should use references too. Pull Request: https://projects.blender.org/blender/blender/pulls/138361