[WPF] Fix popover positioning with top arrow position
[WPF] Fix popover positioning with top arrow position

The popover should not obscure the content it is poping under.
This matches the behavior of the Mac backend.

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