Add scene preferences

[kaascevich]

This PR:

• Adds a ScenePreferenceValues type, which does exactly what it says on the tin
• Refactors the command system to use this type
• Refactors _App somewhat to accommodate this change

[stackotter]

Thanks for doing this refactor. Everything looks great; my requested changes are all minor

[stackotter]

Tested locally and all seems to work well. I only tested with AppKitBackend, because the changes are all platform-agnostic.

The application menu successfully gets updated on app state updates, and app-level state updates in general still work.

[stackotter]

Just fixed the merge conflict from merging the window behaviour modifiers PR and all looks good now. I'll merge once CI passes