Separated sprite loading function so it can be used by SpriteGroup.
Fixed bug (seems more like "bad decision") where if a Tilesheet was created as opposed to loaded for Sprite and SpriteGroup, it would have an additional reference because they would add ref to the tilesheet when it already had one ref (from creation).
Added ability to load SpriteGroup via Properties file.
Fixed bug in SpriteGroup where if a child was set and then replaced by "default" or "transparent", it would still be referenced and wouldn't be freed.