[foldcode addon] Make sure DOM widgets are cloned before being used

So that the addon doesn't try to put a single DOM node in two places

Closes #4848

addon/fold/foldcode.js

@@ -65,6 +65,8 @@
       widget = document.createElement("span");
       widget.appendChild(text);
       widget.className = "CodeMirror-foldmarker";
+    } else if (widget) {
+      widget = widget.cloneNode(true)
     }
     return widget;
   }