Similar to previous changes for masks and visibility, remove the mutable
pointer to face sets from the sculpt session. With implicit sharing,
this avoids a duplicate copy of face sets on the evaluated object when
face sets aren't edited.
Most retrieval of face sets now goes through the Mesh attribute API
instead of the pointer in sculpt session. This integrates sculpt code
with the rest of Blender, avoiding the need to reinvent things like
basic attribute access.
Similar to previous refactors to mask and visibility operators, refactor
the face set gesture and edit operators to be multithreaded and only
push undo steps and tag updates for actually changed nodes.
There is opportunity for more code deduplication in the future.
For now the aim is to reduce the number of confusing abstraction
levels though.
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