When this class creates an object, it sets the object's embedded
gadget.width and gadget.height fields according to the frame image in
ga_image.  if the ga_image understands the im_framebox method, the gadget
asks the GA_Image what it dimensions would be if it had to surround the
display element.  If the GA_Image does not support IM_FRAMEBOX, it just
copies the GA_Image image's width and height into the gadget structure.