layers initial impl (no sensors)

src/core/stack.rs

@@ -6,7 +6,13 @@ pub struct Stack {
 
 impl Widget for Stack {
     fn draw(&mut self, painter: &mut Painter) {
+        let base = painter.layer;
         for child in &self.children {
+            if painter.layer == base {
+                painter.layer = painter.child_layer();
+            } else {
+                painter.layer = painter.next_layer();
+            }
             painter.widget(child);
         }
     }