Use the existing sorting functionality in SfxItemPool and extend it to
search for NameOrIndex item in SvxUnoNameItemTable

This is a little tricky in that we are defining only a partial ordering
over the CntUnencodedStringItem (and their subclasses) items.
Partial because I can only use the part of
the item that is not randomly mutated by various code, which is why
the other fields in the subclasses are mostly out of bounds.

I had to exclude FillBitmapItem because it triggers a unit test
failure and I cannot figure out why that specific item does
not play nice with this optimisation.

After this optimisation, the load time goes from
3.6s to 2s on my machine.

Change-Id: I52d58c68db2536b69a7b0a9611a2b4c703bc4928
Reviewed-on: https://gerrit.libreoffice.org/71461
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>