'Metro' style for 'Frame' control

Styles the 'Frame' control to look like 'MetroNavigationWindow'.

This is part 1 of 2, the second part will provide a helper to receive
navigation events within a 'MetroWindow' just as it is the case for a
'MetroNavigationWindow'.

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Hello,

This is my first contribution to the project, it styles a `Frame` to a 'Metro' look so it can be used within a `MetroWindow`.

My main motivation was the fact that while `MetroNavigationWindow` is indeed cool, you cannot instantiate it in XAML (https://github.com/MahApps/MahApps.Metro/issues/1599#issuecomment-60123909). I've copied the style from `MetroNavigationWindow` and adjusted it for it to work inside the style.

Now since `MetroWindow` is based on `Window`, it does not receive navigation events, On the next update I will provide a helper to be able to receive navigation events as it is the case with `MetroNavigationWindow` or WinRT apps.

I think I forgot nothing related to the integration of that into the project, let me know otherwise.

Thanks :D