TreeModel: Inform view about changed root

This removes the need to emit the respective signal by the caller
of setRootItem() if the tree has been constructed before using
setRootItem().

Change-Id: I6f6210a9ef7d9ec3d347dc7cfdaf853535ca7656
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/utils/treemodel.cpp

@@ -967,6 +967,7 @@ void TreeModel::setRootItem(TreeItem *item)
     delete m_root;
     m_root = item;
     item->setModel(this);
+    emit layoutChanged();
 }
 
 void TreeModel::setHeader(const QStringList &displays)