[WPF] Minor sizing fix for BoxBackend
[WPF] Minor sizing fix for BoxBackend

Looks like after all an UpdateLayout call is required after arranging
each child.

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