Blender Git Loki

UI: Add icons for new face set tools

This patch adds missing icons for the new sculpt face set tools, and
updates the mask icons to be more consistent with the changes.

Currently draw face sets and draw mask icons are inconsistent and don't
relate to drawing/painting masks or face sets. This commit makes the
icons consistent and reusable for future tools like sculpt vertex
colors.

Differential Revision: https://developer.blender.org/D8875