Remove WindowManager wrappers.
It's not safe to use WindowManager in all places and it's not required.
Environment stores resource system providing VFS required to call these
functions. In the case of ObjectPaging it's available from the member variable.
Also ObjectPaging::createChunk may access WindowManager when it's already
destructed when exiting the game because it's destructed before CellPreloader
finishes all background jobs. Engine::mResourceSystem is destructed after all
other systems so it's safe to use it.
