Fixed Switcher widget

This was broken in unreleased changes, hence no changelog update.

src/widgets/switcher.rs

@@ -55,7 +55,7 @@ impl WrapperWidget for Switcher {
         context: &mut LayoutContext<'_, '_, '_, '_, '_>,
     ) -> Size<ConstraintLimit> {
         if self.source.has_updated() {
-            self.child.unmount_in(context);
+            self.child = WidgetRef::new(self.source.get());
         }
         context.invalidate_when_changed(&self.source);
         available_space