Windows: Release mouse capture when window is blocked by modal window.

The capture needs to be cleared when for example a modal dialog
is opened from a timer slot or similar while moving the window.

Task-number: QTBUG-43308
Change-Id: Id0c01080d67d1057004a7f85b037dce5e220de42
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1770,6 +1770,8 @@ void QWindowsWindow::windowEvent(QEvent *event)
     case QEvent::WindowBlocked: // Blocked by another modal window.
         setEnabled(false);
         setFlag(BlockedByModal);
+        if (hasMouseCapture())
+            ReleaseCapture();
         break;
     case QEvent::WindowUnblocked:
         setEnabled(true);