[w3c/manifest] Install event definition is missing some details (#475)

The [section on the install event](https://w3c.github.io/manifest/#installation-events) seems incomplete.  For example, what is the type of the event (I assume just `Event`?), what are its `cancelable` and `bubbles` properties set to?  Is there a default action? 

The spec doesn't even seem to say explicitly that there is a DOM event called "install" outside the example (technically the `oninstall` attribute is separate).  But the intent is clear.

See, for example, the [load event](https://w3c.github.io/uievents/#event-type-load) for a more completely defined DOM event.

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