avm2: Add deep_copy method to XmlListObject

Also changes existing AS3 copy method to use that instead.
Moved as other code needs to do the same operation.

core/src/avm2/globals/xml_list.rs

@@ -201,12 +201,7 @@ pub fn copy<'gc>(
     _args: &[Value<'gc>],
 ) -> Result<Value<'gc>, Error<'gc>> {
     let list = this.as_xml_list_object().unwrap();
-    let children = list
+    Ok(list.deep_copy(activation).into())
-        .children()
-        .iter()
-        .map(|child| E4XOrXml::E4X(child.node().deep_copy(activation.context.gc_context)))
-        .collect();
-    Ok(XmlListObject::new(activation, children, list.target()).into())
 }
 
 pub fn attribute<'gc>(

core/src/avm2/object/xml_list_object.rs

@@ -97,6 +97,15 @@ impl<'gc> XmlListObject<'gc> {
         self.0.read().target
     }
 
+    pub fn deep_copy(&self, activation: &mut Activation<'_, 'gc>) -> XmlListObject<'gc> {
+        let children = self
+            .children()
+            .iter()
+            .map(|child| E4XOrXml::E4X(child.node().deep_copy(activation.context.gc_context)))
+            .collect();
+        XmlListObject::new(activation, children, self.target())
+    }
+
     pub fn equals(
         &self,
         other: &Value<'gc>,