[w3c/manifest] More restrictive standalone mode (#770)

I'm playing with Edge canary on desktop and `standalone` mode is bloated with UA specific UI (translation button, password manager button, back button) 
![edge_standalone_ui](https://user-images.githubusercontent.com/11896237/60395378-b6429e00-9b32-11e9-9c8b-5bbb1cbd2c1c.jpg)

https://www.w3.org/TR/appmanifest/#display-modes
Problem is, the `standalone` description is very permissive:
> the user agent will exclude standard browser UI elements such as an URL bar, but can include other system UI elements such as a status bar and/or system back button.

minimal-ui says:
> This mode is similar to standalone, but provides the end-user with some means to access a minimal set of UI elements for controlling navigation (i.e., back, forward, reload, and perhaps some way of viewing the document's address). A user agent can include other platform specific UI elements, such as "share" and "print" buttons or whatever is customary on the platform and user agent.

On desktop, I expect a basic OS window from the `standalone` mode but the spec definition can lead to a `minimal-ui-light` mode.
So, Is it possible to be more restrictive about the `standalone` mode ?
For example, keep invasive system UA (back button) hidden by default

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