Using symbolic icons in extensions

[Tim Cuthbertson <tim gfxmonk net>]

I've been trying to figure this out for a while, and it seems like it
can't be done.

1) I have got symbolic icons working in a shell extension panel
button, but *only* by setting the icon_name property on a St.Icon,
which requires icons be installed in a system location.

2) If I want to use an icon contained in my extension's folder, I have
to create a GIcon based on that path, and then assign it to the
St.Icon's `gicon` property.

It seems I can't have both. Am I missing something? There doesn't seem
to be a way to construct a symbolic FileIcon, nor a way to
programmatically add my extension's icon location to the system icon
lookup-by-name mechanism. Surely it must be possible, can anyone help?

Cheers,
 - Tim.