Improve Notifications to display more meaningful information

(Split from https://github.com/nextcloud/news-android/issues/999)

It looks like the notifications were already improved in the past (https://github.com/nextcloud/news-android/issues/754). I'd take this one step further and actually show the headlines of the articles in the notification, just like other news apps do. The "You have unread items" message is just forcing you to expand the notification to get some meaningful information and even then it's just the bare minimum (headlines). When the notification is expanded, you could show each article one by one and the source feed or the first words of each article for example.