Extract conversations from conversation notification style

Messenger apps such as Telegram, Riot or Whatsapp often use the "Conversation" notification style to show multiple messages in the notification. Right now we can only show the last message in the notification.

This patch extracts the messages from the conversation and concatenates them so we can show them all.

Now, when receiving e.g. three consecutive messages in a Telegram group all are shown in the popup instead of just the last one