ExTreeView: Fixed missing first root element
ExTreeView: Fixed missing first root element

Manually updating the widget layout only for root elements, as children are not affected by this bug

This fixes https://bugzilla.xamarin.com/show_bug.cgi?id=58917
This closes #720

lytico/xwt,TheBrainTech/xwt,hamekoz/xwt,mono/xwt,antmicro/xwt,hwthomas/xwt,cra0zy/xwt