Blender Git Loki

Fix T47371 - add access to 'static' enum items.

Some dynamic enums, which do not need a valid context pointer, have their 'itemf'
callback always called. This is annoying for introspection tools (like the ones generating
translations, or API documentation), because it means they never have access to all possible
options (enum items).

So now, there is also an `enum_items_static` accessor to get only statically-defined
enum items.

Note: only i18n tools take advantage of this currently, others are still to be updated.

Reviewers: campbellbarton, sergey

Differential Revision: https://developer.blender.org/D1782