Rethink some boolean types

[cjcodeproj]

There are a few boolean attributes floating around in the schema which might make more sense if they were set up as enums.

Two examples that come up are this:

 <case slipcover='true'>

and

 <variantTitle sortable='true'>

In both of these cases, it makes more sense to have flexible attribute types that can store one or more different values from a series of choices. Using the above examples, we could have....

 <case features='slipcover'>

and

 <variantTitle traits='sortable'>

The goal of the ticket is to examine some of the booleans in use, and see if it makes more sense to turn them into custom types for situations where we don't want to fall into a trap of multuple true/false values within a single element.

See also: #15