sd: stop calling graphics styles "Image Styles"

(regression from a71a562a9958fc58a5fb914bcf7f8056a353f618)

Change-Id: Ie3a44a80c720f59831bc2b1da91792fb8aa24d62
(cherry picked from commit b55b96589f7fe0b2f426c2472da7174ab95aab87)
Reviewed-on: https://gerrit.libreoffice.org/10307
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>