[w3c/manifest] Rewrite installation process and install prompting logic (#790)

This change (choose one):

* [ ] Breaks existing normative behavior (please add label "breaking")
* [ ] Adds new normative requirements
* [ ] Adds new normative recommendations or optional items
* [X] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Commit message:

Rewrite installation process and install prompting logic per #788.

- Introduces a new concept of the document being "installable", a stateful
  property of the document based on validity of the manifest.
- Adds a new process, "steps to determine installability of the document", split
  out from "steps to install the web application".
- Removed the concept of "installation process", "installation succeeded",
  "installation canceled" (which was unused; see #787) and "installation
  failed". These concepts are now part of the "steps to install the web
  application".
- "Steps to install the web application" now assumes that the document is
  installable, and that user has already accepted the install prompt, where
  previously the determination of installability and prompting were all part of
  that same process.
- Processes that show an install prompt now explicitly call "steps to install
  the web application" after the user accepts the prompt (see #786).

Closes #786, #787, #788.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/790

-- Commit Summary --

  * Rewrote install prompts and installation process.
  * Fix usage of in parallel.
  * Clarify that install prompt may only be shown if the document is installable.

-- File Changes --

    M index.html (220)

-- Patch Links --

https://github.com/w3c/manifest/pull/790.patch
https://github.com/w3c/manifest/pull/790.diff

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