[w3c/manifest] Add option to replace title bar with a display a smaller caption control overlay (#847)

Developers want the ability to edit the title bar in desktop apps to give a native-like feel. However, out of concern of malicious sites spoofing the title bar and caption controls in the title bar, some of the controls in the title bar still need to exist. 

The proposal is to add a new optional boolean member to the manifest, `caption_controls_only`, which works in conjunction with `display: standalone` to display the caption controls and other browser controls required for security.

![image](https://user-images.githubusercontent.com/8602218/73408558-7488fa00-42b1-11ea-8cab-9e3a86cd269f.png)

See explainer: 
https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/master/TitleBarCustomization/explainer.md

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https://github.com/w3c/manifest/issues/847