Ensure that icon-only buttons get the 'image-button' style class

Due to the way Gtk.Button works, when a Gtk.Image is added via the
'child' property the button does not get the 'image-button' style
class attached to it. We need to add it via the 'image' property for
that to happen.

https://bugzilla.gnome.org/show_bug.cgi?id=734620