This converts the public `uiItemL` function to an object oriented
API (`uiLayout::label`), 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/138608