Support multi-select for TreeView

[HppZ]

[lindexi]

@HppZ See https://stackoverflow.com/a/18751667/6116637

And https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit , C#/WPF - Multi-select TreeView - YouTube

[miloush]

I support this to be a built-in feature of WPF. TreeView and ListBox at least should inherit from MultiSelector.

[HppZ]

https://stackoverflow.com/a/18751667/6116637
there are problems when virtualization is enabled for TreeView.