This PR adds react-native-windows support to react-native-video. The Video component is implemented with a XAML MediaPlayerElement. Most of the features implemented by Android (and some additional ones) are implemented by Windows.
Known issues and missing features include:
* onReadyForDisplay event
* local URI convention (e.g., "broadchurch" in examples changed to require("./broadchurch.mp4")
* `playableDuration` in `onVideoProgress` event is always 0.0
* `playInBackground` is not yet supported
* Volume settings are applied, but the UWP control does not handle it properly
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