Add window.postMessage communication from web view to web view provider/controller

[tjcouch-sil]

User Story
As an extension developer, I want to be able to use built-in browser APIs to communicate from my web view to my backend so I can embed normal websites and further sandbox my web view to protect it from other web views.

Description

As of #1185, we have a way for web view provider/controllers to postMessage to web views. However, web views cannot do the same to communicate back to the backend. They can communicate on the papi, but they can't use built-in browser APIs. As such, URL web views and web views not on same origin are unable to talk to their backend.

This could be useful for syncing scripture reference with external sites embedded into P.B, keeping track of which url a website has navigated to, and much more. It depends on how the website creator implements postMessage.

Implementation ideas: Web view postMessage calls receiveMessageFromWebView on web view provider? Or on web view controller? Or WebViewFactory works with call on web view provider to make it easier for web view controllers? Or something along these lines. Probably use the web view nonce to make sure the web view provider receives messages only from the web view.

[tjcouch-sil]

#1185 and this implement #616.

[Sebastian-ubs]

I have heard interest in this for an easy start of existing scripture reference based web apps. Compared to putting the app itself into Platform, this would work with just the following

• create a new url webview - with watching on bcv changes (might be made simpler with my upcoming changes) - or using this platform post message api
• in the web app add calls to this platform post massage api on bcv changes