The widgets `NotificationMessage` and `NotificationSection` in `calendar.js`
aren't used only by the calendar.
Move the two widget to messageList.js since once we add by-source grouping for
messages (which will happen in a future commit) we need a much tighter
coupling between them and the rest of the MessageList. In future the
`NotificationSection` will need to be removed to make expanding of
groups work.
This also removes a circular import of files: `calender.js` imports
`messageTray.js` and it imports `calender.js`.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3429>
64aadeece3