Consistency around SvxB3DVectorItem in svx/svx3ditems.hxx

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

Change-Id: I7f2348d82c76f6f231e34f0dfc4b6ee6fddffe55