Album thumbnails don't scale as nicely as image thumbnails when the window's width changes

Spun off from https://bugzilla.gnome.org/show_bug.cgi?id=740425

The GtkFlowBox-based grid nicely scales image thumbnails when the window's width changes. The same doesn't work for albums. It only increases the width of the black border on the left and right edges, and doesn't scale the smaller sub-thumbnail images.