[w3c/manifest] purpose: badge should be defined as default badge for Notifications API (#508)

The `purpose` property is weakly defined. The only current value, `badge` is told to be for "where space constraints and/or color requirements differ from those of the application icon" but the characteristics of the icon are not known by the UA and the restrictions depend on the platform.

Shouldn't we define `badge` as the default icon for the `badge` property from the Notifications API unless one is specified? Also say that UA can re-use it in situations where an icon following notifications' badge restrictions would make sense? Hopefully, that could help both UA and developers to make the right decision.

Note that `badge` in general is a very confusing term because AFAIK, [Microsoft](https://msdn.microsoft.com/en-gb/windows/uwp/controls-and-patterns/tiles-badges-notifications) and [Apple](http://i.stack.imgur.com/EgJcX.png) use this term for stuff like unread count. Though, I guess the Notifications API picked this name and we are stuck with it.

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