[w3c/uievents] Add a definition of `contextmenu` event. (#316)

Closes #279

The following tasks have been completed:

 * [ ] Confirmed there are no ReSpec/BikeShed errors or warnings.
 * [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

All major browsers already fire `contextmenu` event.  The "commitments" below really indicate the "already-shipped" status, so bug links are irrelevant.  Here is a [test page](https://codepen.io/mustaqahmed/full/Pojrdeo).

 * [ ] WebKit
 * [X] Chromium
 * [X] Gecko
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/uievents/pull/316

-- Commit Summary --

  * <a href="https://github.com/w3c/uievents/pull/316/commits/3a6e6adc03e046124a1a191ed51da36b52d72b86">Add a definition of `contextmenu` event.</a>

-- File Changes --

    M sections/event-types.txt (57)

-- Patch Links --

https://github.com/w3c/uievents/pull/316.patch
https://github.com/w3c/uievents/pull/316.diff

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