This converts the public `uiItemO` function to an object oriented
API (`uiLayout::op`).
Also this rearranges `idname` paramether, since this the only one
required, and to make format similar to `uiItemFullO`
Note: One of the benefits of moving from a public function to class
method is to reduce API usage difference between C++ and Python. In
Python this method is called `UILayout::operator`, however `operator`
is a reserved keyword in C++.
Part of: #117604
Pull Request: https://projects.blender.org/blender/blender/pulls/138776
a017a6cc54