MultiSelectTreeViewItemAutomationPeer doesn't handle children inside of Grid or StackPanel

[jaysacco]

If a MultiSelectTreeViewItem contains controls (e.g. CheckBox or ComboBox) which are laid out inside objects that don't have AutomationPeers (e.g. Grid or StackPanel), the controls will not be reported as children of the MultiSelectTreeViewItem. This means they can't be used in automated tests.

I have a fix for this and will be submitting a pull request.