Consistency around SfxBoolItem in svx/svx3ditems.hxx

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

Change-Id: I3253b4cc5657a7d6b960ee892584109d373eed3d