[core] Only mark unread items as read
[core] Generalize markAllAboveAsRead()

This is needed when using a RemorseItem on sailfish as the selected
item might have changed between starting the RemorseItem and its
execution.
[sailfish] Added RemorseItem to markAllAboveAsRead()
[sailfish] Don't catchup but mark all loaded as read when in a feed

It's still possible to catchup by using the context menu of the feed.