[Tree] Allow for the complete repopulation of a Tree. #1437
Description
Slightly related to #1396 in that to do it now the ability to change a TreeNode.label would be required. At the moment it is possible to use Tree.clear to clear all the nodes under the root node, but there appears to be no public interface for totally clearing a Tree and repopulating it from scratch -- other that performing a remove on the widget itself and then a mount of a fresh instance.
A use case for this would be if the Tree is being used to view the content of a file in a structured way, and you want the user to be able to load in a new file.