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.
Ah, right. On windows and posix there are two invocations with the same size (windowIcon, tryIcon).
Let me implement a simple mem-cache or see what Qt provides here.