Enable more fine-grained control over screen locking handling

[siy]

This PR enables convenient and correct handling of screen timeouts using code like this:

    // Application main loop
    loop {
        let event = comm.next_event();

        match event {
            Event::Ticker => {
                if UxEvent::Event.request() != BOLOS_UX_OK {
                    UxEvent::block();
                    // Redisplay application menu here
                }
            }
            ...
        }
    }