Consistency around SfxUInt32Item in svx/svx3ditems.hxx

...similar to what has been done for svx/sdtmfitm.hxx in
68969cc61adecac481ae9656978ef952f435b310 "Consistency around SdrMetricItem."

Change-Id: I3193eab34a34c051002adeedd8b368e26f55f7a3