Provide a default icon

[Tim Cuthbertson <tim gfxmonk net>]

I'm currently writing a gnome-shell extension that I want to add panel
icons to. None of the default icons are appropriate, so I have created
my own.

Now, I could (presumably) hard-code the path to my default icons, but
that's bad form because then no theme can possibly override them. So I
believe I should instead use an icon name, one that doesn't already
exist. I'm thinking "tiling-window-layout-horizontal" as an example.

What files to I have to put in what path (I'm thinking $XDG_DATA_DIRS
will come into it) so that:
 - my icon will *always* be available, regardless of the user's preferred theme
 - the user's preferred theme can provide their own implementation of
"tiling-window-layout-horizontal"

Note: I can't change files in system directories, I'm looking for
something I can make happen via an environment variable.

Making an icon set doesn't sound right, because then the user would
have to select it. And adding files to a default theme also sounds
brittle, because they would then presmably not be used if the user
picks a different theme. Unless there is a hard-coded default
fallback, but even then can you add a single file to a default without
modifying system directories?

Thanks,
 - Tim Cuthbertson