Consider de-duplicating icon files to reduce file size

[Rob--W]

The current size of all icons in extension/icons/ is 706369 bytes.
If duplicates are discarded, the size drops to 345732 bytes.
(source: 384413e)

Consider adding a build step that removes duplicate images and adds aliases (e.g. in extension/mime-metadata.js via gen-mime-info.js) to reduce the size of the extension package.