Fix TreeView virtualization.

[walterlv]

I have a TreeView with more than 10000 items. When I click to expand an item it expands in more than 5 seconds. So we have to add the virtualization supports for the TreeView.

Pull request type

Please check the type of change your PR introduces:

• [ ] Update
• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes

What is the current behavior?

The performance is very poor.

Issue Number: 1

What is the new behavior?

• When we enable the virtualization using VirtualizingPanel.IsVirtualizing="True", it can now works in a better performance as the original WPF TreeView does.

Other information