Add Properties.Canvas to Channel

[ku]

Not documented in the reference of conversations.info but now the response of conversations.info contains Canvas related data when the channel has a canvas for the channel. ( properties key itself is not contained if no canvas is associated to the channel)

{
    "ok": true,
    "channel": {
      .....
        "properties": {
            "canvas": {
                "file_id": "F05T9HXD8DQ",
                "is_empty": true,
                "quip_thread_id": "MCE9AA8oXgH"
            }
        }
    }
}

A Canvas dedicated to the channel can be created by the button placed in top right of the window.