feat: Avatars in notifications for Linux for all desktop envs

I use a minimalist notification manager (dunst) and while it can display images, it does not work if the "icon" is a Gdk.Pixbuf, which I guess is GNOME-specific. Passing a Gio.FileIcon does work though and that's what this commit does.

It requires writing the icon to disk, but it seems we can easily cache that using the hash of the cairo.Surface object, so I don't think the performance impact is going to be an issue.