[Mac] Fix size calculation for extracted Custom Widget
[Mac] Fix size calculation for extracted Custom Widget

CustomWidgetBackend will wrap its child in a WidgetPlacementWrapper
if the child has any margins set. The wrapper will inset the child
by the appropriate amount, but we also need to add that amount to
the total measured size of the Custom Widget

hamekoz/xwt,mono/xwt,TheBrainTech/xwt,lytico/xwt,steffenWi/xwt,mminns/xwt,akrisiun/xwt,antmicro/xwt,hwthomas/xwt,iainx/xwt,residuum/xwt,mminns/xwt,cra0zy/xwt