icon-utils: add a method to embed a surface in a frame

It's a variant of gd_embed_image_in_frame() that takes a cairo_surface_t
instead of a GdkPixbuf.
gd_embed_image_in_frame() now is implemented on top of the surface
variant.

Note: while it may seem that this makes framing a pixbuf slower by
adding a pixbuf->surface conversion, GTK+ already implements
gtk_render_icon() on top of gtk_render_icon_surface() so it ends up
being the same.