AppStream: update for new guidelines

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
All screenshots should have a 16:9 aspect ratio, and should have a width that is no smaller than 620px (this will ensure that your screenshots correctly fit into the space provided by the software application). Ideally the window will be resized to a 16:9 aspect ratio, but screenshots can also be cropped if only a small area of the window needs to be shown.