load-more-button: Simplify the callback
Ensure that icon-only buttons get the 'image-button' style class

Due to the way GtkButton works, when a GtkImage is added via
gtk_container_add the button does not get the 'image-button' style
class attached to it. We need to add it via gtk_button_set_image for
that to happen.

Fixes: https://bugzilla.gnome.org/737310