theme: Use a more generic method for laying out path bar buttons

The fix for Nautilus path bar changing its size due to width differences between bold and regular text involves packing a bold label and a regular label into a GtkStack and switching between them. This means that the "label:first-child" and the "label:last-child" selectors are no longer reliable since they now reflect the order of the labels in the GtkStack and not whether the label comes before or after the image.