Prevent modification to a label

[bogovicj]

• I would like to be to specify that a particular label is "unmodifiable" or "immutable"
• I should not be able to modify the label of any voxel if that voxel's label is "immutable".
• It would be nice to have a short cut to make all label "immutable"

Example workflow:

• Make all labels immutable
• Make one label mutable (A)
• Create a new label
• Paint with the new label
  • painting in this state is fast because existing boundaries between label A and others are preserved.

I'd call this feature label "locking" but there is currently a feature that "locks" a label, but it seems to only affect how it is displayed. I'd call the current functionality "hiding".