native Gnome file dialogs don't get the focus

How to reproduce:

• build and run qtbase/examples/widgets/dialogs/standarddialogs
• Press "QFileDialog::getExistingDirectory()".
• Note how the dialog gets focus correctly.
• Close the file dialog.
• Press "QFileDialog::getExistingDirectory()" again.
• Note how the dialog does NOT get focus anymore.

After the first time, the file dialog will never get focus when it's opened.

The situation is worse if there's a full-screen application, for example the gnome terminal, running in the background: after pressing the "QFileDialog::getExistingDirectory()" gnome-terminal will get the active window and you'll have to Alt-Tab back to your application.

Using non-native dialogs doesn't show this behavior.
Non-Qt apps that use native dialogs don't show this behavior.