Update to support new banner design from gnome-software 40
We’ve just landed a new banner design in gnome-software 40. Now a banner is auto-generated from an app using its icon, key colour from the icon, and title and summary. CSS-customised banners are still supported, but deprecated in favour of the style.
Banner Viewer should potentially be updated to be able to render the new style as well as the old one.
See GNOME/gnome-software#1131 (closed) and GNOME/gnome-software!621 (merged).