Individually, these don't make much difference, but they add up
to a halving the time to save on my machine.

ManifestImport::characters was spending time adding data to an OUString,
so convert that to an OUStringBuffer.

Change-Id: I267e701f4e7998044763f44199b1fe8a37325b68
Reviewed-on: https://gerrit.libreoffice.org/70311
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>