feature request: support adding new tree elements in the UI

[travis-bear]

It would be nice to be able to add child elements -- both new branches, and new leaf nodes.

This could possibly be achieved with a context menu, or by little "+" buttons.

[jledentu]

Hi, the itemComponent prop allows to pass a custom component for items. Your custom component could handle this context menu or + button that would trigger the creation of a new child item. Then just pass a new tree value.

Since it's not so difficult to add this feature on your side, and since I couldn't provide a UI that would satisfy all users, I consider it's out-of-scope.

I'm still open to a PR that would suggest a generic solution. :)