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.
1)Cache
do you think kind of a file cache for the resized image? The hue shifted testnet icon could also be cached, but it's kinda dev only things where some used cputicks maybe are tolerated. But filecaching it brings us a little bit back to the apple recommended @2x strategy. Maybe the distributed binary is not unnecessary bloated, but we use disk space (very minor indeed).
I think the rescaling from 1024 to 256 does perform very fast and implementing a cache with correct invalidating mechanism (for possible upcoming icon changes) might need some time.
But tell me if I'm sounding lazy, then i go behind it.

Reimplementing QIcon.pixmap
Ah. Now i see what you mean. Yes. I try to optimize this. It was more or less "organically grown" and than i oversaw the quick way resizing the standard icon. Thanks for that.