implement .clone, .slice, and yxml.get

src/types/YXmlElement.js

@@ -8,7 +8,7 @@ import {
   typeMapGetAll,
   typeListForEach,
   YXmlElementRefID,
-  AbstractUpdateDecoder, AbstractUpdateEncoder, Snapshot, Doc, Item // eslint-disable-line
+  AbstractType, AbstractUpdateDecoder, AbstractUpdateEncoder, Snapshot, Doc, Item // eslint-disable-line
 } from '../internals.js'
 
 /**
@@ -55,6 +55,20 @@ export class YXmlElement extends YXmlFragment {
     return new YXmlElement(this.nodeName)
   }
 
+  /**
+   * @return {YXmlElement}
+   */
+  clone () {
+    const el = new YXmlElement(this.nodeName)
+    const attrs = this.getAttributes()
+    for (const key in attrs) {
+      el.setAttribute(key, attrs[key])
+    }
+    // @ts-ignore
+    el.insert(0, el.toArray().map(item => item instanceof AbstractType ? item.clone() : item))
+    return el
+  }
+
   /**
    * Returns the XML serialization of this YXmlElement.
    * The attributes are ordered by attribute-name, so you can easily use this