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.
1 job for path-bar-stack-fix in 39 minutes and 26 seconds (queued for 10 seconds)