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.
@jonasschnelli Nonono, a file cache shouldn't be necessary. I just like to avoid doing the same scaling multiple times within the same invocation of Bitcoin-Qt. I think QIcon already implements such functionality to maintain the image at multiple sizes, so there would be no need for us to do anything complex here.