Add an API for armature layer access. Instead of accessing `arm->layer`
and friends directly, the code now uses this API. This will make things
easier to replace by bone collections in the future.
The functions are named "bonecoll" (short for "bone collection"), as
that's the soon-to-be-introduced replacement for armature layers. This
API is the first step towards that replacement, and should help to
reduce the changes necessary when functional changes are committed.
This also creates a new module `source/blender/animrig` for Animation &
Rigging code. This will, for example, house the bone collection system
in the near future.
There is a bunch of code currently spread across blenkernel and editors
in a rather ad-hoc way; it is intended that at some point that code gets
moved into `animrig` as well (or at least the subset of that code where
such a move makes sense; brain still required).
Ref: #108941
No functional changes.
Sybren A. Stüvel