Add support for enum values in ID properties.
This is needed for the "Menu Switch" node implementation (#113445) which
relies on ID properties for the top-level modifier UI.
Enums items can optionally be added to the UI data of integer
properties. Each property stores a full set of the enum items to keep
things simple.
Enum items can be added to properties using the `id_properties_ui`
function in the python API. A detailed example can be found in the
`bl_pyapi_idprop.py` test.
There is currently no support yet for editing enum items through the UI.
This is because the "Edit Property" feature is implemented entirely
through a single operator (`WM_OT_properties_edit`) and its properties.
Buttons to add/remove/move items would be operators changing another
operator's properties. A refactor of the custom properties UI is likely
required to make this work.
Pull Request: https://projects.blender.org/blender/blender/pulls/114362
92cf9dd2f2