[AFV] Ref T731, allow to re-init audio in UI

* audio can be started/stopped and disabled/enabled
* allow to connect signals if audio was started
* allow default devices in combobox

src/blackmisc/connectionguard.h

@@ -57,6 +57,9 @@ namespace BlackMisc
         //! Disconnect all
         int disconnectAll();
 
+        //! Empty
+        bool isEmpty() const { return m_connections.isEmpty(); }
+
     private:
         QList<QMetaObject::Connection> m_connections;
     };