Implement read markers

Read markers are implemented as specified by XEP-0333: Chat Markers. Only the "displayed" part of the XEP is implemented.

Read markers are sent when a message marked as "markable" is received and displayed to the user or marked as read via a notification's action. When read markers are received, a text is shown in the corresponding chat. Additionally, the counter for unread messages is decreased and notifications are closed if read markers for messages from a contact are received from another own client. That snychronizes the state of read messages between multiple endpoints.

Closes #299 (closed), #367 (closed)

Co-authored-by: Melvin Keskin melvo@olomono.de