This commit introduces a new mode for calculating the positions and
weights of the IK segments in the Pose Brush based on the Face Sets.
The first segment of the chain will always include all face sets inside
the brush radius and it will propagate until the boundary of the last
face sets added in the flood fill. Then consecutive connected face sets
are added to the chain until the chain length limit is reached or all
face sets of the mesh are already part of the chain.
This feature enables complete control over the pose brush origins in
case that is needed. Also, with this mode, the user can have a library
of base meshes with face sets already configured to get to the initial
pose as fast as possible.