[Feature]: Mark All Messages as Read

### Precondition - [x] I understood that this template is only for feature requests and not for bug reports ### Cross-checked related issues? - [x] I have cross-checked this overview https://github.com/monal-im/Monal/issues/322 as well as filtered for related labels https://github.com/monal-im/Monal/labels ### Describe your feature Monal does not appear to have a "Mark all as read" function on the MacOS Desktop App. This would be useful because I use a variety of XMPP applications on different devices, and my unread vs read messages don't appear to sync to Monal. When I'm away from Monal for a while the number of unreads can stack up. Potential Solutions: A simple, small button above the chat list on the left side of the app may be a good place for this. Alternatively, this could be an option when right clicking a chat on the left side. Thanks team! Love the app.