[manifest] Support for Actionable Notifications in the manifest (#347)

### Introduction

I wanted to add support for actionable notifications for a web app, that can be discovered using its manifest. The use case behind this is to be able to support the [Physical Web project](https://github.com/google/physical-web)**. As we have more sensors (or IoT devices) which essentially expose simple services, it is not scalable to have to install an app for each and every IoT device or sensor being used. This is why the Physical Web project is using `Bluetooth LE` and `mDNS` to expose services offered by these devices as web-apps that can be discovered using a common protocol.

### Why and How

The manifest should be able to support actionable notifications as an enhancement, for quick actions. Quick actions are links to more things (URL's or deep links).They give fast 1-click access to frequently used features in the web-app (without having to visit the web app). These 1-click actions could then be exposed via by the user agent using Notification panels or custom UI components; thus making them readily available.

The Website's manifest file can include quick actions that are supported by the site.
This is what the the `payload` in the manifest could look like:

    [
      {
        "icon": "icon_url",
         "title": "The Title of the actionable Notification",
         "link": "deep_link_or_url"
      },
     {..}
    ]

**Related discussion in the [Physical Web] (https://github.com/google/physical-web/issues/307) project.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/347