Qt: HiDPI (retina) support for splash screen

Currently the splash screen is not optimized for retina/HiDPI screens. This changeset will enable high-resolution background images and text drawing. This should also work on other platforms than OSX as soon as they support HiDPI.
Wow :/
I wonder if this could be  solved by explicitly adding a 256x256 scaled bitmap to the icon in the `NetworkStyle::NetworkStyle` constructor using `void QIcon::addPixmap ( const QPixmap & pixmap )`. That at least moves the code away from here.