[w3c/manifest] Add an icon "purpose" for displaying on the theme color (#617)

Problem: We sometimes want to display the app icon on top of the app's theme color (e.g., for a window icon in a title bar that is themed to the theme color). Many brands will want to use a theme color that exactly matches the dominant color of their icon, which will look bad when applied on top of the theme color.

For example, Facebook's icon is just a white 'f' in a blue square. If they set their theme_color to the same blue, then when the user agent places the icon on top of the themed title bar, it will just look like a floating white 'f' in a sea of blue. Facebook works around this problem in their Android app by providing a special inverted icon with a blue 'f' in a white square. There is no way to specify a special purpose icon like that in a web manifest.

I propose simply adding a new [purpose](https://www.w3.org/TR/appmanifest/#purpose-member) called "`on_theme_color`" (bikeshed away) that lets you specify a separate icon asset specifically for painting on top of the theme color.

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