Fix the path bar changing size when navigating

This was due to size differences between bold and regular labels.

This patch fixes this by making the existing label bold temporarily
to measure how wide it is when it is bold instead of bothering with an
invisible bold label (I couldn't make this approach work).
1 job for no-jumpy-pathbar-conservative-2 in 12 minutes and 6 seconds (queued for 11 seconds)
Status Name Job ID Coverage
  Test
passed flatpak:master #19561

00:12:06