Add insert/delete styles to image cells in unified diffs

In patch sets that add or delete an image, only one image is shown.
Since there was no background styling, one had to look at the headers
to find out whether the image got added or deleted. We now set the
correct styles, and therefore looking at the cell's background
suffices to see whether it was an addition or deletion.

For pictures, where the styling gets in the way (e.g.: partly
transparent pictures against white background), styling can by turned
off in the same way as for text.

Change-Id: I5341ce1a587c5ae98118831bfef5f61143935ec6
1 file changed