winrt: Fusion should be set as the default style

The windows style is enabled on winrt by default for historic reasons (it being a Windows platform), but it doesn't really make any sense there. As the GDI calls are not available, it doesn't even really look like Windows Vista/7 (and it certainly doesn't look Metro).

For those reasons, the Fusion style should be the default used on WinRT, as it provides a cleaner, nicer looking, and more generalized look. Even though it is not a touch style, it looks more touch friendly than the basic style.