[manifest] Ability to define platform-specific icon style (#361)

Different platforms have different guidelines regarding their icon style, and an app may need to follow them to look "native". 

Wrong style, even if slightly wrong, can create "uncanny valley", e.g. multiple OSes use square icons with rounded corners, but with a different radius. Icon with a wrong corner radius looks like an amateurish knock-off.

The spec currently only matches icons on size and density, but not style/theme, so sites would need to resort to user-agent sniffing if they wanted to serve system-specific icons.

A theoretical solution available in HTML, with help of [Moz's nonstandard media queries](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries), is:

    <link rel=icon href="generic.png">
    <link rel=icon media="all and -moz-windows-theme:luna-blue" href="xp.png">
    <link rel=icon media="all and -moz-windows-theme:aero" href="vista.png">

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/361