TressData item copy/move enhancements

[fschon]

TreeData currently has methods to remove and addItems as well as a moveSibling method. The removeItem and addItem use the fluid pattern and return the entire TreeData. When removeing an item all of its children are also removed, and lost.

There is no method to arbitrarily copy an item from one place in the tree to another, together with its children. The moveSibling is restricted to items with the same parent.

Additionaly when adding an item, the add only appends the item at the end of the tree. There should be a way to to prepend the item as well. This is needed in a Drag and Drop implementation using TreeGrid and TreeData - similar to the example here When dragging a item onto a different parent, the item can only be added to the bottom of the child list, whereas the BETWEEN indicator suggests it should be added to the beginning of the child list.

See forum post here.