MaterialDesignToolToggleListBox with RadioButton behaviour

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## Background
I'm using the `MaterialDesignToolToggleListBox` for selecting one of up to three possible modes that can be available to the user. The current implementation of `MaterialDesignToolToggleListBox` will allow the user to unselect items in the list box which leaves us with no item selected.

### Current Behaviour

Since i'm binding the `SelectedValue` of the `ListBox` I will get a binding/validation error since the `SelectedItem` will be `null` when the user unselect the item.

### Expected Behaviour

## Solutions
I would like to set a property on `ListBoxAssist` or use a new style to get a `RadioButton` behaviour on `MaterialDesignToolToggleListBox`.

1. Create a new style `MaterialDesignToolRadioButtonListBox` which uses `RadioButton` to achieve the desired result. (I have not tried this)
2. Use an attached property to hook into the `SelectionChanged` event and prevent unselect. Possible name could be `PreventUnselect`

*Pseudo code for option 2*

```
private void SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if(sender is ListBox listBox)
{
if (ListBoxAssist.GetIsToggle(listBox) &&
listBox.SelectionMode == SelectionMode.Single)
{
if (e.AddedItems.Count == 0 && e.RemovedItems.Count == 1)
{
listBox.SelectedItem = e.RemovedItems[0];
}
}
}
}
```

Start by locating MaterialDesignToolToggleListBox and ListBoxAssist, then inspect how toggle selection handles SelectionChanged for single-selection ListBox controls. Compare the proposed style and attached-property approaches; done means the selected item cannot be cleared, while SelectedValue binding remains valid.

Written by the indexing model from the issue text.