Furniture Sprites

Started implementing Furniture sprites, discovering, incidentally, that if a sprite's image cannot be found, Jade crashes. Must fix that.

Though Furniture sprites are INACTIVE (i.e. don't react to touch or collision), they sometimes have to be moved to and from Limbo, so their 'home' scene and location are remembered. Initially, I set up special variables to remember these, but it occurs to me that I could use a single Dropzone for the purpose. Saves coding.