Partially implement fingerprint display

This requires asynchronously observing events, and we might need to put an async Lock around the QObject now. Either way, the asynchronous event observer should not borrow the QObject across await points, especially since the QObject RefCell does not correctly implement borrowing rules.

Contributes to #513. It does not fix it, because I currently have no way to trigger the Qt signal from the observer actor. This will need refactoring again, and I want this in and released asap now.

TODO:

• Implement a Qt signal that can be called from the ObservingModelActor to signify asynchronously loaded properties.
• Allow pushing verification page and message page from group overview (fixes #516 (closed))
• Get rid of the extra Recipient{} in the group overview page (use the Augmented Recipient type in the recipient list model)
• Do some tests with session reset
• Opening message view from group overview does not work when there's no existing session. We can't easily make this session before opening it, I think, so we should disable this menu item when there's no current session (session = -1).