don't add subtree of a document to parent document and small performance improvement

Source/SvgElementCollection.cs

@@ -84,11 +84,15 @@ namespace Svg
             	if (this._owner.OwnerDocument != null)
             	{
             		this._owner.OwnerDocument.IdManager.AddAndFixID(item, autoFixID, logElementOldIDNewID);
+            		
+            		if(!(item is SvgDocument)) //don't add subtree of a document to parent document
+            		{
             			foreach (var child in item.Children)
             			{
             				child.ApplyRecursive(e => this._owner.OwnerDocument.IdManager.AddAndFixID(e, autoFixChildrenID, logElementOldIDNewID));
             			}
             		}
+            	}
 
                 item._parent = this._owner;
             }

Source/SvgExtentions.cs

@@ -57,11 +57,13 @@ namespace Svg
         {
         	action(elem);
         	
+        	if(!(elem is SvgDocument)) //don't apply action to subtree of documents
+        	{
         		foreach (var element in elem.Children)
         		{
-        		if(!(elem is SvgDocument))
         			element.ApplyRecursive(action);
         		}
         	}
         }
+    }
 }