Re: [manifest] Allow developers to detect when the user has added the site to their home screen (or equivalent) (#417)

> Let's add it then.

"It" being "onbeforeinstall", allowing UAs to not necessarily make us of this. This event is cancellable, does not bubble, etc. Chrome folks, let me know what you do here exactly and I'll spec it.  

And "install" event, happens on successful "installation" (i.e., whatever that means to the UA, as to where it allows the user to access the "installed" web application). This event is not cancellable, does not bubble, etc. 

> Does the "isInstalled" boolean have to always be 100% up to date or could it be a cached value?

I say we punt on `isInstalled`. It just won't be reliable. 

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