[Xwt.Wpf] Fixed GetPrevious Xwt.WPFBackend.
Merge pull request #357 from mono/fix_wpf_getprevious

[Xwt.Wpf] Fixed GetPrevious Xwt.WPFBackend.
[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
Merge remote-tracking branch 'upstream/master'
[GTK3] include missing project file reference