Call ::repaint explicitly on the QSplashScreen after showing it.

This ensures that we actually see the configured pixmap instead of
an ugly grey rectangle. The reason is apparently that at this stage
we do not have an event loop running. See also QSplashScreen::repaint
documentation:

> This overrides QWidget::repaint(). It differs from the standard repaint
> function in that it also calls QApplication::flush() to ensure the
> updates are displayed, even when there is no event loop present.