Implement the pause overlay (closes #29)

Assets/Scenes/Simulation.unity

Assets/Scripts/Presentation/UI/BattleUI.cs

Assets/Scripts/Presentation/UI/PauseOverlay.cs

Assets/Scripts/Presentation/UI/SimulationUI.cs

Assets/Scripts/Presentation/World/SimulationManager.cs

Assets/UI/DialogStyles.uss

Assets/UI/DialogStyles.uss.meta

Assets/UI/Simulation/BattleUI.meta

Assets/UI/Simulation/BattleUI/BattleUI.uxml

Assets/UI/Simulation/BattleUI/BattleUI.uxml.meta

Assets/UI/Simulation/PauseOverlay.meta

Assets/UI/Simulation/PauseOverlay/PauseOverlay.uxml

Assets/UI/Simulation/PauseOverlay/PauseOverlay.uxml.meta

Assets/World/Structures/Towers/EmptyTowerType.asset

Assets/World/Units/EmptyUnitType.asset