Feature/ocsadv 23

These changes address the GUI issues brought up in OCSADV-23, disabling the unbounded position angle option for MOS imaging and custom mask FPU observations (and FPU = None).

Also, this fixes a bug with respect to the parallactic angle controls appearing when they shouldn't.
(See REL-2089: http://jira.gemini.edu:8080/browse/REL-2089)
I'm guessing this was tricky to get working.  The visual effect is pretty cool and seems like something that should be in the library already.  I just wonder if it is worth maintaining this vs. changing the model to only show available items.  It's pretty late for a comment like that I know.  I guess the question is whether the value added by this widget minus the expected maintenance cost is greater than the alternative of only showing available items.  I don't know the answer actually.