Detachable webview

[arsher]

I've been looking into how to create detachable webviews, or better said something that can transfer the content into another webview instance. It might be a bit hacky currently, I'm pretty sure it's not prod quality yet, but I wanted something working to start the conversation.

I've seen #1682 which led me to the conclusion that I'm not alone with the desire to have something like this, hence this PR.

[deepak1556]

Nice! Just a few thoughts, could add WebViewManager::TransferGuest(sourceGuestInstanceId, targetElementInstanceId, targetGuestInstanceId) and let guest-view-manger use it to change mapping of guestinstanceid to embedder. Also ipc operations in web-view can be moved to guest-view-internaland ones' in web-view-ref can be handled by guest-view-manager. Not sure about the need for WebViewRef class.

[anaisbetts]

@arsher This is awesome, good start!

[zcbenz]

Can you add some documents for it?

[arsher]

Sure thing, I'm sorry I had about zero amount of free time, I'll try to do some docs and responses for the comments during the weekend.

[alexandernst]

Is this the same thing as atom/atom#9334 ?

[zcbenz]

I'm closing this since there were no updates and the PR is outdated, but I'll be glad to reopen once there are new updates.