Adding retracted legs sprites for the Suspension Field Generator (xenoarch)

So we get a visual feedback that the Generator is fixed or not. Not to mention, we get a message telling that "the stabilising legs are wrenched into place" or "up against the body" but the sprite itself doesn't change at all.
![legs](https://cloud.githubusercontent.com/assets/7573912/2964553/c28138e0-dae7-11e3-9162-00a346cb3673.png)
(there are also ones for when the cover is opened or the cell is missing)