[w3c/manifest] Fix(installation): clarify installation event (closes #473) (#474)

 * Drop mixin in favor of partial interface
 * Clarify that installation is an `Event`. 
 * Link to steps to fire the event 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/474

-- Commit Summary --

  * Fix(installation): clarify installation event (closes #473)

-- File Changes --

    M index.html (22)

-- Patch Links --

https://github.com/w3c/manifest/pull/474.patch
https://github.com/w3c/manifest/pull/474.diff

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