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.

(cherry picked from commit 70f15298)

fix windows build (from Noel Grandin)

(cherry picked from commit 4c5498ec)

Change-Id: If99a3efc4727a07df9d2daaefbdacc9565920005
Reviewed-on: https://gerrit.libreoffice.org/17051Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>