Added StageManager class, so now Editor and GameMechanics (aka Level) are kinda equal by code level, Editor does not interact with Level, nor wise versa
Renamed GameMechanics to GameLevel
Editor level loading working, but still buggy
Editor: resolved addToStage problem
Refactoring finished, now we have basic StageManager class and two derived ones: GameLevel and Editor
No errors, but not working
Merging finished