#8 Support hiding of root node in IUnTreeView

[Rene Zanner]

In some applications the root node of a tree view does not contain vital information and must be excluded from view, so that the tree view starts with the first child level.

Status quo:
-----------

ROOT
+ child 1
child 1.1
child 1.2
child 2
+ child 3
+ child 3.1
child 3.1.1
child 3.2

Goal:
-----

IUnTreeView.setRootVisible(false);

+ child 1
child 1.1
child 1.2
child 2
+ child 3
+ child 3.1
child 3.1.1
child 3.2

This feature requests corresponds to javax.swing.JTree where the property "rootVisible" hides the root node of the TreeModel when set to "true".