Add many doxygen comments

src/game/core/Engine.hpp

src/game/core/Scene.hpp

src/game/core/Spritesheet.hpp

src/game/core/Tile.hpp

src/game/effect/Effect.hpp

src/game/entities/Building.hpp

src/game/entities/Unit.hpp

src/game/level/Level.hpp

src/game/player/Player.hpp

src/game/ui/TileMarker.hpp

src/game/ui/context/ContextMenu.hpp

src/game/ui/context/RecruitingMenu.hpp

src/game/ui/menu/EndScreen.hpp

src/game/ui/modals/HelpMenu.hpp

src/game/ui/modals/UnitInfoMenu.hpp