Replace placement calls with region nodes

core/src/widget/data.rs

@@ -3,6 +3,7 @@ use crate::Widget;
 pub struct WidgetData {
     pub widget: Box<dyn Widget>,
     pub label: String,
+    pub(super) region_node: bool,
     /// dynamic borrow checking
     pub borrowed: bool,
 }
@@ -16,6 +17,7 @@ impl WidgetData {
         Self {
             widget: Box::new(widget),
             label,
+            region_node: false,
             borrowed: false,
         }
     }