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

Ok, `install` is growing on me. Basically, `install` translates to successfully dropping an icon + name of app somewhere. 

I support what @owencm is saying: being able to check, in the sense of a boolean, if an app is installed is going to be very hard (if not impossible) to implement on various OSs... as @owencm also points out, it would need to be done async, because the user could trash the icon right after install - or at any point... if at all possible, the UA would need to somehow check if the icon it dropped on the homescreen is still present (which means at least IO or IPC somewhere). 

I also understand the value of `onbeforeinstall` and us our implementation on Android ramps up, it will likely mean that Fennec will need this event also: however, I don't know yet how our Android UX team will want to implement the install flow for a web app (i.e., if they will need it at all, or if they will do something similar to Chrome). I hope I can answer that in the next few months. 

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https://github.com/w3c/manifest/issues/417#issuecomment-175969070