[manifest] Desktop web apps should be able to declare their viewport size (#317)

**Real life use cases:**
- Cut The Rope is designed for a 3 to 4 viewport ratio and looks best at 640 by 480. If opened in a different ratio, the game will be letterboxed which should be avoided.
- Wunderlist is optimized for using minimal screen estate and looks blown up when resized too far. To look best on first launch, a developer should be able to specify recommended window size.

Display modes that mimic the windowing behavior of native apps, `standalone` `minimal-ui`, should optionally allow declaration of a `viewport` property to set initial app window viewport size.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/317