Blender Git Loki

Animation: new pose library based on Asset Browser

Introduce new pose library, based on the Asset Browser. Contrary to the
old pose library (in `editors/armature/pose_lib.c`), which stored an
entire library of poses in an `Action`, in the new library each pose is
its own `Action` datablock. This is done for compatibility with the
asset browser, and also to make it easier to attach preview images,
share datablocks, etc. Furthermore, it opens the door to having
animation snippets in the pose library as well.

This commit contains the C code for the pose library; in order to fully
use it, an addon is required as well (which will be committed shortly).