Support for platform hints on icons

[owencm]

Looking at Slack's website the other day I noticed just how different icon standards are for different platforms:

I wonder if we should support the ability for sites to optionally hint a specific icon is more suitable to a certain platform. For example the manifest could provide default icons in different sizes in the way today, and additionally versions with a new property like suggested_platforms containing a list of strings corresponding to platforms that icon would be well suited for.

I'm not sure of the best way for us to standardize (or leave unstandardized?) platform strings though so UAs know which OS to apply them on - naively we could define a short list and add ones as they request to be added, but that seems potentially difficult. Also it's unclear to me whether we'd want to support versioning (e.g. 'win10' vs 'win' etc).

[marcoscaceres]

@owencm thanks for the additional input + example! This is a duplicate of #361. Please take a look.

FWIW, we are planning on adding platform. Just waiting for Gecko to catch up.