In the good ol' name of memory efficiency, Phobos now removes scenes
that aren't need, i.e:-
	PreloaderScene removes itself once it adds the Main Menu
	Submenus of the Main Menu remove themselves when back is selected.
	LevelZero (branching scene) removes all the other scenes upon loading.
	
Removed removeScene, added removeTopScene and removeLastScene - added
convenience methods to Engine to reflect these changes.