This simplifies the C++ API for making layout panels. Now it is also more similar to the Python API.
`uiLayoutPanel` is like `layout.panel` and `uiLayoutPanelProp` is like `layout.panel_prop`.
Both, `uiLayoutPanel` and `uiLayoutPanelProp`, now exist in two variants. One that takes a label
parameter and one that does not. If the label is passed in, only the panel body layout is returned.
Otherwise, the header and body are returned for more customization.
Pull Request: https://projects.blender.org/blender/blender/pulls/117670
f358843108