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.
*BTW* you are reimplementing part of the QIcon functionality here. See QIcon.pixmap(QSize).
Apart from the hue-shift part, but that could be done once on instantiation?