Add a property `openDownloadsListOnStart` to the download object prototype. It is true by default. When passed a false value, it will prevent the usual behavior of opening the downloads panel when the download starts. This will allow some (forthcoming) internal code in the WebExtensions API to prevent showing the downloads panel when a download is created without any user input (see bug 1759231 for details). Differential Revision: https://phabricator.services.mozilla.com/D142503