Fix path bar changing size when navigating

This was due to size differences between bold and regular labels. The
problems was that invisible widgets always return 0 when their size is
requested and Nautilus tried to request the width a regular label would
take from an invisible bold label with the same text. This patch fixes
this by showing the label before measuring it and hiding it afterwards.
1 job for no-jumpy-pathbar-conservative in 10 minutes and 11 seconds (queued for 12 seconds)
Status Name Job ID Coverage
  Test
passed flatpak:master #19531

00:10:11