When we get a WindowBlocked event then reset the qt_button_down value

Whenever a window gets blocked, then the qt_button_down variable should
be reset as it is not going to receive the release event for the mouse.
This fixes problems with the mouse cursor not being updated when moving
over widgets (such as QLineEdit) after a dialog is shown in response to
a press event.

Change-Id: Idfd072eaf36f51b816a0b2a0391cdc447d7a5d9d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/kernel/qwidgetwindow.cpp

@@ -251,7 +251,9 @@ bool QWidgetWindow::event(QEvent *event)
     case QEvent::Show:
     case QEvent::Hide:
         return QWindow::event(event);
-
+    case QEvent::WindowBlocked:
+        qt_button_down = 0;
+        break;
     default:
         break;
     }