Certain members of ItemAutomationPeer should be public

Certain members of `ItemAutomationPeer` should be public to enable people to write better code by avoiding code copies or usage of reflection.
The members in `ItemAutomationPeer` especially affect custom automation peers for `ItemsControl` like controls.
Those internal members are, for example, used in `RibbonGroupDataAutomationPeer`.

Members that should be public:
- `GetWrapper`
- `GetWrapperPeer`

Exposing those members would remove the need to use reflection to call those methods, as their code can't be easily copied as `GetWrapper` relies on the internal type `IGeneratorHost`.