GUI changes for best position angle feature / parallactic angle feature.

The following changes are UI only for OCSADV-23 and subtasks. The logic for best PA for guiding have yet to be completed.
Do we use UNKNOWN for anything in particular?  Could we remove it and just use UNBOUNDED?  That is, are the two the same in practice?  If we use it in a situation where we really don't know what the constraint should be, then maybe the type we want there is Option<PosAngleConstraint>?