Add responses in document for layer system (#91)

core/document/src/layers/folder.rs

@@ -47,6 +47,10 @@ impl Folder {
 		self.layer_ids.as_slice()
 	}
 
+	pub fn layers(&self) -> &[Layer] {
+		self.layers.as_slice()
+	}
+
 	pub fn layer(&self, id: LayerId) -> Option<&Layer> {
 		let pos = self.layer_ids.iter().position(|x| *x == id)?;
 		Some(&self.layers[pos])