Windows: Use focusWindow() as native dialog parent when 0 is passed.

A parent is required, otherwise the application gets deactivated when
the dialog closes. The same is done in Qt 4.

Task-number: QTBUG-36039

Change-Id: I56688b3259f1d207edd1bec499afd1365ae511dc
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

src/plugins/platforms/windows/qwindowsdialoghelpers.cpp

@@ -572,6 +572,8 @@ bool QWindowsDialogHelperBase<BaseClass>::show(Qt::WindowFlags,
                                                    QWindow *parent)
 {
     const bool modal = (windowModality != Qt::NonModal);
+    if (!parent)
+        parent = QGuiApplication::focusWindow(); // Need a parent window, else the application loses activation when closed.
     if (parent) {
         m_ownerWindow = QWindowsWindow::handleOf(parent);
     } else {