Created singletonize decorator to reduce duplicated code written in the graphx singleton
Added event manager that is used to register callback to call on event or key combinations. Game already uses it, but other class should soon use it either. This will allow to replate all the handleEvent and handlePressed functions by simple eventManager.registerEvent() during construction
Now using the eventsManager for all the events of the game. Also fixed the bug that happened when unregistering an event in an event callback