[Vala] Setting image on a button?

[Damien Radtke <damienradtke gmail com>]

I'm trying to add an image to a button, but it doesn't display. I'm loading
one of the stock images, and can get it to display fine on its own, but it
doesn't show after placed on a button with set_image(). I've also looked
into the "gtk-button-images" setting to see if it's disabled, but
Gtk.Settings doesn't seem to support that property.

Using vala 0.11.5.