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 23 minutes and 14 seconds (queued for 8 seconds)
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