icon-helpers: Improve thumbnailing style

Thumbnails are now square to avoid reflow when loading new items which
might have mixed types of media (vertical videos, covers vs. horizontal
TV/cinema videos).

Mockup is at:
https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/videos/thumbnails-square.png

https://bugzilla.gnome.org/show_bug.cgi?id=743247