fix (ladder): Make sure price-range 'busy' is flipped back to false

Once the async price-range-shift op is completed
Bugs (fixed):
  * Can't scroll beyond initial price range in orderbook ladder

  Cause: The LadderGridHost's price-range-shift 'busy' flag gets stuck to false

src/client/spark/Modules/Trading/Ladder/View/LadderGridHost.cs

@@ -573,6 +573,7 @@ namespace mbts.spark {
             }
             busy = true;
             await dispatcher.InvokeAsync(ensure_tick_padding, DispatcherPriority.Background);
+            busy = false;
 
             void ensure_tick_padding() {
                if (_price_list is null || _settings is null) {
@@ -601,7 +602,6 @@ namespace mbts.spark {
                   _pending_price_list = new_pl;
                }
                _ordershost.refresh_orders();
-               busy = false;
             }
          }
       }