scenarioRef should be a non-optional member of UIGameplay

[kostmo]

scenarioRef should be a non-optional member of UIGameplay, and uiGameplay should be an optional member of UIState.

This would encode the invariant that UIGameplay pertains to an actively-running scenario. That is, the uiGameplay member should be Nothing when not playing a particular scenario (i.e. in the toplevel menu), and a Just UIGameplay while playing. With that, we should be able to guarantee that the UIGameplay record always has a non-null reference to a Scenario.

This would simplify #2264.

[byorgey]

This makes a lot of sense. Currently, every time we start a scenario we manually reset all of the UIGameplay fields within the UIState to default values (see scenarioToUIState:

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

Lines 266 to 283 in f5c0700

	& uiGameplay . uiDialogs . uiGoal .~ emptyGoalDisplay
	& uiGameplay . uiIsAutoPlay .~ isAutoplaying
	& uiGameplay . uiFocusRing .~ initFocusRing
	& uiGameplay . uiInventory . uiInventorySearch .~ Nothing
	& uiGameplay . uiInventory . uiInventoryList .~ Nothing
	& uiGameplay . uiInventory . uiInventorySort .~ defaultSortOptions
	& uiGameplay . uiInventory . uiShowZero .~ True
	& uiGameplay . uiTiming . uiShowFPS .~ False
	& uiGameplay . uiREPL .~ initREPLState (u ^. uiGameplay . uiREPL . replHistory)
	& uiGameplay . uiREPL . replHistory %~ restartREPLHistory
	& uiGameplay . scenarioRef ?~ siPair
	& uiGameplay . uiTiming . lastFrameTime .~ curTime
	& uiGameplay . uiWorldEditor . EM.entityPaintList %~ BL.listReplace entityList Nothing
	& uiGameplay . uiWorldEditor . EM.editingBounds . EM.boundsRect %~ setNewBounds
	& uiGameplay . uiDialogs . uiStructure
	.~ StructureDisplay
	(SR.makeListWidget . M.elems $ gs ^. landscape . recognizerAutomatons . originalStructureDefinitions)
	(focusSetCurrent (StructureWidgets StructuresList) $ focusRing $ map StructureWidgets enumerate)

), but this is largely duplicated in initUIState. The one exception to this (that I have noticed; there could be others) is the REPL history: we want to be careful to preserve that across scenarios.

Note we could also get rid of the _uiPlaying field in UIState: whether we are playing a scenario would just be determined by whether the uiGameplay is Nothing or Just. If we want, we could still define a uiPlaying Getter as a convenience.