[manifest] Convert icon object to image object (close #327) (#358)

* this is just "editorial" change - it doesn't change any processing aspects of icons or anything like that.  
* generalizes icon to "image" so we can reuse image objects in other contexts, like for splash screens.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/358

-- Commit Summary --

  * Convert icon object to image object (close #327)

-- File Changes --

    M index.html (214)

-- Patch Links --

https://github.com/w3c/manifest/pull/358.patch
https://github.com/w3c/manifest/pull/358.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/pull/358