Implement "UNDO"

CommandType enum add .Undo

IFacade add Undo method

Implement Undo in (concrete) GameFieldFacade
 when PopAt is called -> inside call SaveState (otherwise it's null)
 when Undo is called ->  check for a valid previousState (not null)
    load the state into GameField

Engine add .Undo case
 catch invalid op exception

ConsoleUI add "Undo" string case

PromptType enum {missingBalloon, unableToUndo}