Separate UI from core

[merces]

Currently, UI and core code are mixed up. I've seen projects where UI is separated from the core. Would it be good? Maybe to connect another framework instead of ratuatui if we want in the future?

[glazari]

This is probably a big effort. But I think I can help go in that direction.

• Tests (since its a big change, having the tests help us make the changes with more confidence)
• Make "draw" only draw and not have mut access to App
• Make the event handler function take our own type rather than crossterm event

Take these with a grain of salt, since I've never done significant UI work.