This sets the unexpanded value, so going to a "cooked" FilePath can
at least theoretically break. On the user side it saves a
roundtrip in a few cases, but is more ugly when the input is already
a proper FilePath.
Change-Id: I8a7e8f6d46fcc34c96b55e41d656dca933fbef4e
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>