Editor: Default values for Fixed Choices should be constrained

[saulgreenberg]

The default value entered in a Fixed Choice row should be constrained to strings in the FixedChoice list (e.g., maybe selected via menu?).

Alternately, put a red box around the default value if it is not in the list.

This is a minor bug, as a default choice that doesn't match what is in the list can introduce inconsistencies.

[saulgreenberg]

Resolved. Default values are cleared and an error message displayed if the default value is not contained in the choice list. This is tested both when the default value or the Choice list is edited