[Refactoring] All refactoring tasks for the moment

[w-i-n-s]

• AppState god object. Injected in every view. Contain every manager/store. Breaks the "single responsibility principle" because it becomes the source of truth for every view.
• Implement MVC or MVVM (with "finite state automata"). Possible with reducers [TBC]
• Take care about error presenting on UI [TBC].
• Make a user action atomic and consistent (App should not show a QR code with invalid stuff in it). App model should be consistent, but without implementing force unwrapping or assertions. Possible using SwiftLint
• (migrated into Implement CoreData persistent store #12) Implementing multi-wallet. Should use Core Data as ORM. Only unique entropyMask can stay in defaults for whole app. For each wallet:
  • which network (testnet, mainnet, and later also Signet)
  • the threshold (how many signatures are required)
  • information about each co-signer: their xpub and derivation path
  • a list of pre-derived addresses
• Change the way how the app starts (AppDelegate and SceneDelegate -> App/WindowGroup iOS14)

[Sjors]

After the v1.0 milestone let's split this into individual issues.