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
Also workaround a nasty bug, where unregistered py classes remain listed in relevant __subclasses__() calls, which would lead to crash with python addons i18n tools (main translation was not affected, as messages extracting tools are executed in a brand new "factory startup" Blender ;) ).
Fix for keyingsets tips, and make them (and a few others) findable by i18n messages extracting code (for some reasons, their bl_rna.description are void???).
Most notable difference from now on will be that all py is handled from current blender's resource dirs, no more from source dir. Better for consistency, and avoid e.g. cycles' addon to be checked twice...
Notes:
* Everything is still a bit raw and sometimes hackish.
* Not every feature implemented yet.
* A bunch of cleanup is still needed.
* Doc needs to be updated too!
