Adding the possibility to insert widgets inside the Tree

[zladivliba]

A very interesting new feature would be to allow users to use the tree as a base source for displaying widgets instead of text nodes.

This would allow to add text + inlineEditBoxes for exemple, show different widgets in orders, ... wild range of possiblilities.

[bill]

This is effectively already supported by the _createTreeNode(). method. Closing as dup of #5719.