imprv: Notifications: Withdraw all when a corresponding chat is read
Fixes #11030 (closed)
The event approach seemed more elegant than reaching for a NotificationBackend
object in some way and making _withdraw()
public.
The check for "unread > 0" added to ChatList.mark_as_read
is only a premature optimization, I haven't profiled sending out withdrawals for a hundred joined MUCs. ;)