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.
for me it only works with this resize.
Otherwise i have the following tray icon in Ubuntu:

![unbenannt-2](https://cloud.githubusercontent.com/assets/178464/5049075/ec57aa6a-6c22-11e4-8be2-a089ac0263f4.png)