Add icons for packages and elements

[justinfagnani]

Fixes #24

This format is a direct copy of PWA manifest icons: https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Manifest/Reference/icons

I'm unsure if we want to recommend any particular icon sizes or not. App stores have pretty large sizes, like 512 x 512.

[bennypowers]

This would be useful for me. not only icons, but screenshots, posters, etc. the "icons" name is fine as long as it allows for but listing icons and larger posters

see https://ux.redhat.com/elements/

[bennypowers]

LGTM

[justinfagnani]

@bennypowers I agree with the need for non-icon images too. I'll update this so that icons are just one of a few particular image roles.

Do you have particular role needs or suggestions? I personally want to enable image carousels/galleries on a catalog page that show previews of the component. I suppose that's a poster? Do we want to have some preferred sizes or aspect ratios?

[justinfagnani]

PWA manifest has screenshots: https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Manifest/Reference/screenshots

I do really like the idea of copying someone else's proven format here.