Fix Item ordering in 'Select*', 'ToggleButtons' and 'RadioButtons' widgets

[jbweston]

Fixes #1191.

I'm not sure if you are still accepting pull requests for bugs on the 5.x branch, but given jupyter/dashboards_server#317 I still need ipywidgets 5.x, so thought I would make this change available upstream.

I essentially change the logic to be more inline with that on the current master branch (example),
where instead of updating the DOM with elements and then deleting any which are no longer present, we instead rebuild the list of DOM elements from scratch.

Now elements in the DOM respect the ordering of elements in the options of the model.

[jasongrout]

Thanks! I'm not sure if we'll be releasing another 5.x release, but this will be in it if we do.

[jasongrout]

Since we now are on ipywidgets version 7, we don't plan any more 5.x releases. Thanks again for your work on this. The dashboards server is now longer active either.