Revert the state of GraphicURL property so it supports setting a
external URL, but getting throws and exception.

Modify the Graphic property, so it reflects what was used with
GraphicURL before.

Change-Id: Ia05b8ea37997b5ed8c75215557cce9979630c898
Reviewed-on: https://gerrit.libreoffice.org/53509Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>