[Xwt] Fixed/Added Tree Operations:
- Xwt.GtkBackend.TreeStoreBackend: Added GetPrevious implementation
- TreeNavigator - Added MoveToFirstSibling/MoveToLastSibling.
- TreeStore - Added InsertNodeAfter/InsertNodeBefore.
[Xwt][GtkBackend] Fixed GetPrevious using TreePath ( lluis suggestion https://github.com/mono/xwt/pull/355#discussion-diff-14774581 )
Merge pull request #355 from mono/fix_tree_operations

[Xwt] Fixed/Added Tree Operations: