When image is set to a Button control the correspoding graphic object
lives only temporarily, after the graphic is copied from the graphic
object it's destroyed so save can't find it by graphic ID.
Use the same solution which works for ImageControls: use a graphic object
member for the control, so save can find it.

Change-Id: If99a3efc4727a07df9d2daaefbdacc9565920005