OSX: provide initial icons for DMG. PList might need cleanup and the ico...

...nset is not final by now.

I had to create own icons. OSX expects something called an "iconset" for app-files. They have to be created based on several images with different resolutions, including high-res versions (say Reting :/ see https://developer.apple.com/library/mac/documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/Optimizing/Optimizing.html).
I couldn't use the logo provided as they have a too low resolution.
Sadly, I can currently only test from within QTCreator. Starting the created app always crashes it. We can look into that later - might be some setup screwed up here on my side