[w3c/manifest] Support window preferred/initial sizes (#436)

There is growing interesting in Progressive Web Apps on desktop, both with Crosswalk, Manifoldjs, but developers are also asking for some support natively in browsers.

Desktop OSes have freeform windows support and with Android potentially getting support for this (http://arstechnica.com/gadgets/2016/03/this-is-android-ns-freeform-window-mode/), I think it is time to look at adding such support to the manifest format.

Crosswalk already supports window width and height.

ChromeApps supports something similar (though it goes through a method)

https://developer.chrome.com/apps/app_codelab_basics :+1: 

```
When the Chrome App is launched, chrome.app.window.create() will create a new window using a basic HTML page (index.html) as the source. You will create the HTML view in the next step.

chrome.app.window.create('index.html', {
  id: 'main',
  bounds: { width: 620, height: 500 }
});
```

https://developer.chrome.com/apps/app_window#type-BoundsSpecification

```
integer (optional) left 
The X coordinate of the content or window.

integer (optional) top 
The Y coordinate of the content or window.

integer (optional) width 
The width of the content or window.

integer (optional) height 
The height of the content or window.

integer (optional) minWidth 
The minimum width of the content or window.

integer (optional) minHeight 
The minimum height of the content or window.

integer (optional) maxWidth 
The maximum width of the content or window.

integer (optional) maxHeight 
The maximum height of the content or window.
```

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