Refactor components further

[wahlp]

75

goals:

• break down the main file into something that looks more like MVC
• the last PR left the state manager reliant on class variables. change it to use instance variables to make it more testable in future

[wahlp]

certainly not a perfect separation of responsibilities between the controller and view, but it feels great to see how much smaller each file is compared to the original main.py

[simon-wg]

I took this opportunity to convert Item and Cost to dataclasses as well, and also added a __add__ to Cost so we don't have to iterate over each key.

But it works well on my machine! Great job again :D

[AnthonyMichaelTDM]

ping me when this is ready to review/merge

[wahlp]

@AnthonyMichaelTDM it's ready

[AnthonyMichaelTDM]

sweet, I'll take a look