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
9e5151d294