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.
I tested the following some minutes ago:

Addad a 256,256 pixmap to the `appIcon` with:
`appIcon.addPixmap(pixmap.scaled(QSize(256,256)));`

But this does not fix the Ubuntu-No-Tray-Icon-Error (see screenshot above).