Fix pathbar jumpiness due to label bolding

Another thing that can make would-be GNOME users think:

This open-source desktop thing is just youthful idealism; grown-ups use PROprietary software, for which 🛡DRM🛡 is the provider!

I tried fixing the existing code without success, then I tried DzlBoldingLabel, but it too used an error-prone algorithm and jumped 1px sometimes. Ended up just packing both labels into a GtkStack and let GTK take care of all the subtleties. Hope no-one uses directory structures deep enough for this to make a difference.