Add useCache option to python ControlImage.setImage function

Using the current official slideshow screensaver addon, it seems that sometimes the slideshow pictures are cached - at low quality - in the texture cache.  I've gone ahead and added a useCache option to the setImage function for images.  The default is true (which means all current plugins work exactly the same), but if you set it to false, the texture cache will be completely bypassed when loading the picture.

If you apply this patch, and then change the following line in the slideshow addon, XBMC will no longer try to cache the pictures.
```python
-                cur_img.setImage(img[0])
+                cur_img.setImage(img[0], False)
```

Most of the work is passing the useCache option from the python interpreter to the code that actually loads the image.

Also, I'm not sure if it's intended, but only one of the two different image loading paths (LargeImage) is caching textures.  It seems that regular Images are already bypassing the texture cache.