Mark certain properties as "conditionally optional"

[Galdoc]

The best way to describe this is with an example.

In the ItemPrototype, the properties icon/icon_size and icons are both conditionally optional -- that is, you need either icons or both icon and icon_size. I don't know of a great way to show that these are linked -- I think it is important to figure out how to do something like that, tho! -- but for right now, just having in the column that says 'required' or 'optional' having a third thing to say -- to wit, conditionally optional -- would be very valuable. Thanks!