This adds id_properties_clear() and id_properties_ensure() functions to RNA structs. This is meant as an initial change based on discussion in review of D9697. However, they may be useful in other situations. The change requires refactoring the internal idproperties callback to return a pointer to the IDProperty pointer, which actually turns out to be quite a nice cleanup. An id_properties attribute could be added in the future potentially. Differential Revision: https://developer.blender.org/D11908