Add classfile parsing

[abigailalexander]

This adds the basics of classfile parsing:

• Parse .bcf file based on filepath passed in via config
• Adds entries to theme based off class and widget name
• Override colours on a widget with class colour, if widget has a class set. If not, fall back to widget props
• Add tests

Currently only colours in the classes are applied. Border and font will be included in the MUI theme later, and other properties handled in another way.

[GregJHarris]

Approved with a few comments that you may want to concider.