Blender Git Loki

Fix T84520: Make the different weight paint code paths exclusive to each other

Before this change, you could have the new sculpt symmetry code and the
older weight paint symmetry code active at the same time. This would
lead to users easily trashing their weigh paint data if they were not
careful when switching between modes.

Now the specific weight paint symmetry code is an exclusive toggle so
the user can't accidentally mirror strokes and vertex groups at the same
time. This also paves the way of supporting Y and Z symmetry in the
future for weight groups mirroring if we decide to add it in the future.

Reviewed By: Sybren

Differential Revision: https://developer.blender.org/D10426