Implement viewedReceipt

Implement viewedReceipt

commit cd29144e81701698092f3334bee0c99c0f77f202
Merge: 14532be f884175
Author: AsamK <asamk@gmx.de>
Date:   Sat Oct 30 12:46:59 2021 +0200

    Merge branch 'master' into dbus_sendviewed

commit 14532befded0004d884f7f0ffa20a11ee5383c08
Author: John Freed <okgithub@johnfreed.com>
Date:   Thu Oct 7 08:24:00 2021 +0200

    update deliveryReceipt to conform

commit a9a6edaf3ff8ac81b1229db294d738f624688a0d
Author: John Freed <okgithub@johnfreed.com>
Date:   Thu Oct 7 07:40:20 2021 +0200

    implement viewedReceipt

    for Dbus and Json -- note that this changes the JsonReceiptMessage
    structure

    update documentation

Closes #764