[Bf-animsys] Face-Map 2.8 Proposal

[Campbell Barton]

Based on discussion with Dalai, I've written a proposal for face-map
integration into 2.8 branch.

https://developer.blender.org/T51675

While some of the topics in the proposal may seem obvious,
I'd like to be sure we're on the right track since I wasn't involved
in the original design.

The only functional change I've made in the branch is having the bone
to object+facemap associations calculated automatically
(setting both per-bone was tedious).

Also face-map custom-data layer has been merged into 2.8.

-- 
- Campbell