[Xwt.Wpf] Fixed Tree operations on WPF
- InsertBefore / InsertAfter was creating the TreeStoreNode with wrong parent.
- Avoid NRE on GetVisibleItem ( ContainerFromItem can return null, even after UpdateLayout call).
Merge pull request #358 from mono/fix_wpf_treeoperations

[Xwt.Wpf] Fixed Tree operations on WPF