Simplify showing the QSplashScreen on the right screen

On a virtual desktop setup the splash screen should open on the screen that the application window is going to open up on. Right now, the code has to

• read the saved geometry from the registry
• make sure that the geometry still intersects with the screen (a screen might have been removed since last time)
• create the splash screen with the right desktop widget as a parent
• check if the desktop is virtual
• if it is, calculate the correct position in the center and move the splash screen there

This could be simplified a lot by providing an API QSplashScreen::showOnScreen(int screenNumber) or moveToScreen().