More User Stories

A user can create a deep hierachy of sprites. For example, a user can create a sprite group called “menu” and add another sub-group called “buttons”:
```
 GROUP : menu
     SPRITE : background.png
     GROUP : buttons
         SPRITE : default.png
         SPRITE : pressed.png
```
A user can move a group of sprites to another group by selecting a group and dragging it over onto a target group in a sprite tree.
Sprite sheet can be exported to CSS. A user can use this CSS to display sprites on a web page using background positioning.