Add "userData" member to TreeNode to facilitate right-click menu handling (among other things)

This wish/enhancement would be useful for those using the default TreeNode/TreeBranch as the data model for a TreeView – the use case is the situation where an icon and/or string is sufficient for rendering in the tree but another type of object is more appropriate for determining (for instance) the right-click menu behavior, or the selection behavior of the tree node.

The "String" override for "setUserData" also allows setting this property via WTKX/BXML files.

Diff file attached to the issue I have running already.