Refactor Split template related code

[pappnu]

Depends on #96 because of type annotations.

Changes:

• Refactors Split template and layout so that they inherit their base classes in a type safe way, i.e. so that they don't change the types of the base classes' properties and functions. This aims to improve the code base's compatibility with static type checking.
• Adds rudimentary support for unified typeline when rendering Split cards, but does not add the required functionality and frame elements to the current normal Split template. This is relevant currently only for Room cards.
• Adds a SplitMod class, which aims to provide basic Split rendering logic for inheriting classes.

I have done limited testing, i.e. checked that a regular and a fuse card still render correctly with the normal Split template, so it's still quite likely that there are bugs in the code.

Please let me know if you'd like some bigger or smaller changes to the approach taken here.