Better API for overriding metadata.

Currently it looks like:
```cs
DefaultStyleKeyProperty.OverrideMetadata(
typeof(Button),
new FrameworkPropertyMetadata(typeof(Button)));
```

The problem is that when overriding caller needs to know the type of metadata registered to avoid:

```
ArgumentException: Metadata override and base metadata must be of the same type or derived type.
```

Overriding default value is common and we could add new API so that the framework figures out what metadata type to use:

```cs
DefaultStyleKeyProperty.OverrideDefaultValue(
typeof(Button),
typeof(Button)); // The framework then calls OverrideMetadata with the correct type.
```