Move GameState and UIGameplay into new subrecord in AppState

[kostmo]

Currently, AppState has a dedicated field RuntimeState for scenario-independent state, and a field GameState for scenario-dependent state. However, UIState contains an awkward mix of active-play-dependent widget states and play-independent states (see #2265).

swarm/src/swarm-tui/Swarm/TUI/Model.hs

Lines 176 to 181 in e7b4d65

	data AppState = AppState
	{ _gameState :: GameState
	, _uiState :: UIState
	, _keyEventHandling :: KeyEventHandlingState
	, _runtimeState :: RuntimeState
	}

When embarking on #2265 I noticed that GameState tends to be accessed often alongside UIGameplay. I propose a new toplevel member PlayingState of the AppState that shall contain both GameState and UIGameplay. Eventually, PlayingState should be made nullable.

This refactoring may be accomplished in phases:

1. Move GameState underneath a new toplevel record PlayingState.
2. Move UIGameplay out of UIState into a second field of PlayingState.
3. Wrap the PlayingState field in a Maybe
4. Make scenarioRef mandatory in PlayingState (scenarioRef should be a non-optional member of UIGameplay #2265)

[byorgey]

Sounds good to me. When we first created the AppState record it seemed like a sensible organization to put all the gameplay stuff in one record and the UI stuff in another. However, I think you're right that the better top-level organizational principle should be "stuff that persists across scenarios" and "stuff that only exists while playing a scenario".