For now just introduce the GraphicExternalLink internally in
ImpGraphic, and use it for the origin URL. In a future patch this
will store additional data about the link.

Change-Id: I7b4edac80d0e71603d37243ff28bcac1b18fdc01
Reviewed-on: https://gerrit.libreoffice.org/52393Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>