nodeRemoveChildren() don't collapse node

I have a command that programmatically replaces all child nodes of some tree node (by `removeChildren()`/`addChildren()`). With new version of Fancytree parent node is always collapsed after executing this command. The reason is that `nodeRemoveChildren()` explicitly reset `expanded` flag. I think it's wrong. It's more logically to explicitly set `expanded` as true, because there are no child nodes and they don't keep any space. But I suppose that the most correct way is don't change `expanded` flag at all.