Review Windows-specific components and prop usage and move into "XXXX.windows.tsx" files

[chrisglein]

Things to look for:

• PlatformColor that relies on a XAML brush
• Any usage of react-native-xaml
• Components that don't have other desktop (macOS) support

The end goal here would be that it'd be easier to tackle something like macOS also be able to use RN Gallery. Could use react-native-web as a relatively low cost "desktop-like" platform to test without taking on all of macOS.

[Saadnajmi]

FWIW, I think an Android tablet emulator or an iPad simulator running standard React Native would be easier to test on so you don't have to bring all of React Native web. At least until macOS shows up