sd: stop calling graphics styles "Image Styles"

(regression from a71a562a9958fc58a5fb914bcf7f8056a353f618)

Change-Id: Ie3a44a80c720f59831bc2b1da91792fb8aa24d62
writerfilter: add unit test for Fraction initialization

... as used by wrap polygon import, see fdo#41068 fdo#76803.

Change-Id: I4467560c2c3bbdab3fc9f8cd7a0c0b415429186c