69a7c834b8
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