Add GridLines property to List/Tree views
convert ListBox.GridLines to bool
since ListBox has no columns and no vertical grid lines
Rename GridLines property to GridLinesVisible
[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 'mono/master' into sevoku/add-list-gridlines

Conflicts:
	Xwt/Xwt.Backends/IListBoxBackend.cs
Merge pull request #338 from sevoku/add-list-gridlines

Add GridLines property to List/Tree views