[Xwt] Fixed/Added Tree Operations:

- Xwt.GtkBackend.TreeStoreBackend: Added GetPrevious implementation
- TreeNavigator - Added MoveToFirstSibling/MoveToLastSibling.
- TreeStore - Added InsertNodeAfter/InsertNodeBefore.
Equals() doesn't work well for tree iters, I'm sorry but you can't use it. A better solution would be to get the TreePath for the iter, decrease the latest index, and get the iter from the path.