Part of incoming refactors in interface layout C++ code, this enables
forward declaring this enum type outside of `layout_interface.cc`. The
enum is renamed as `ItemInternalFlag` and moved to the `blender::ui`
namespace. `uiItemInternalFlag` is preserved as a shorthand for
`blender::ui::ItemInternalFlag`.
No user visible changes expected.
Pull Request: https://projects.blender.org/blender/blender/pulls/136782
69a7c834b8