[Bf-funboard] Critical improvements and features

[Campbell Barton]

> 6. Bone weight painting is not reliable. Blur tool produces
> some explainable peaks and has very limited control. There should at least
> be option to blur only already assigned vertex groups and do not assign any
> new. (This is the behavior in Maya). This is important so no other bones
> are assigned to vertices without user knowing it.

This can be done for any brush type, enable 'Restrict' in the tool
options panel.