Currently "ImageURL" property is used to transport the Graphic
(as an string URL), which is being replaced by transporting
XGraphic directly. There is already a property that all
controls use - "Graphic", so use that instead.

Change-Id: I2514b475735f3e430c2baa9b1685a231fc694b28
Reviewed-on: https://gerrit.libreoffice.org/50139Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>