This renames `UI_block_layout` API as `blender::ui::block_layout`,
following uiLayout refactors.
This function now returns a layout reference instead of pointer,
this changes applies this return type where the layout can be used
as such reference.
Changes includes the use of `blender::ui::LayoutDirection` and
`blender::ui::LayoutType` as typed enum parameters.
Part of: #117604
Pull Request: https://projects.blender.org/blender/blender/pulls/141401
b36e5b4b06