Open more API functions for TreeView, e.g., GetItem

I saw this function (**GetItem** in C++ code, https://github.com/isl-org/Open3D/blob/master/cpp/open3d/visualization/gui/TreeView.cpp). It will be great if you could open this function for python users in development version.

As I understand that this will return the CheckableTextTreeCell widget if the CheckableTextTreeCell widget was added, right? If yes, I can get the label of the widget directly. Thanks!

_std::shared_ptr TreeView::**GetItem**(ItemId item_id) const {
auto item_it = impl_->id2item_.find(item_id);
if (item_it != impl_->id2item_.end()) {
return item_it->second->cell;
}
return nullptr;
}_