Remove implementation details from vcl/settings.hxx

This reduces the number of files which need to be recompiled
after a change to the internal details.

Also: Remove explicit assignment operators since they are no longer
needed.

Change-Id: Ieb18a8437fbcd39c6003b50c656ce010f5002d9a
Reviewed-on: https://gerrit.libreoffice.org/8161
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>