Create/Edit MyPaint brushes

Submitted by Jehan (@Jehan)

Link to original bug (#768772)

Description

So I understand that MyPaint brush are not derived from the same parent as other brush tool. Hence (probably) they have no dynamics support. But for a tool supposed to target digital painters, this sucks and makes it a lot less attractive.

I had feedbacks from 2 painters trying the MyPaint brushes, and that's the first thing they asked me about: why tablet sensitivity doesn't work?

Do we plan to have this before 2.10 release or is the feature really considered good-for-release as of now?

Version: git master