qml: use Estimating as the default string for remainingSyncTime

Author: johnny9

src/qml/components/BlockClock.qml

@@ -171,6 +171,6 @@ Item {
             return "~" + seconds + (seconds === 1 ? " second" : " seconds") + " left";
         }
 
-        return "~0 seconds left";
+        return "Estimating";
     }
 }