core (widgets_handler): use a StencilView + BoxLayout instead of ScrollView as wrapper: The default wrapping when no Carousel or ScreenManager was used was a ScrollView. This was causing bugs as the touch event is not propagated correctly when a ScrollView is in children of ScrollView (notably causing trouble with ModernMenu on ContactList and FileSharing widgets). To avoid that, a simple StencilView with BoxLayout is now used instead. If a ScrollView is needed, it can be added inside the widget (this is the case for WidgetSelector).