Bindable "Value" property in DataTrigger.cs

Often you want to bind also the value in a DataTrigger in XAML.

Example usage:
```

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In this I only want to check if the value of a slider in a CustomControl reaches the minimum value of my defined "Minimum" DependenyProperty in my CustomControl.cs to show an indicator element in the ControlTemplate of the CustomControl in XAML.