Ability to freeze specific label assignments in GUI

[nclack]

Two options (choose one):

1. freeze by label id. e.g. all curves with label 1 are write protected
2. freeze by curve. e.g. this (fid,wid) is write protected.

Option 1 seems best. Simple to communicate the locked status to the user since it can be displayed with the state. Option 2 requires some curve-wise indication of the locked state. Option 1 allows application of a lock to groups of curves, whereas option 2 does not. The whisker-by-whisker locking can be effected via Option 1 by using special labels.

Update docs required