Add an markup extension to bind standard values of a type

A long time ago I wrote [Enumerate](https://github.com/YohDeadfall/Enumerate) which is designed to provide standard values of any type if it supports it. It's used mostly for `enum`s, but as I said it isn't limited to them. And because WPF is open source now, I think it's time to make the markup extension built-in so everyone will be able to use out of the box.

## Features

1. Providing values of an `enum` as is.
2. Providing values of an `enum` from `DescriptionAttribute`s.
1. Providing values of any type whose `TypeConverter` returns the standard values via the `GetStandardValues` method.

## Usage:

```csharp

public enum ViewModelStatus
{
[Description("Offen")]
Open,
[Description("Geschlossen")]
Closed,
[Description("In Arbeit")]
InProgress
}

public class ViewModel : INotifyPropertyChanged
{
public ViewModelStatus Status { get; set; }
}

```

```xaml

```

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Related to #455.