[manifest] Add background_color to represent the content's background color (#394)

I would like the Manifest to have a ```background_color``` field so a website could specify its background colour. The expected value is similar to ```theme_color```.

The use case for this is to be able to create a view for a web app with the right color before the app is loaded. For example, on a mobile device, if you add to your homescreen a website, it might take some time before it renders, in which case, a plain white screen wouldn't be ideal. This is different from a splashscreen because if a website is working offline, there might still be a delay between the user clicking and the website being rendered because of starting new processes and loading the data from the disk. In such situation, a splashscreen would be too heavy but showing the backrgound color early would be a smooth transition.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/394