Use cpack to create osx's dmg

Additionally, ensure proper installation of sounds and zonebg resources.
This by now is needed in order to create a working OSX package, but can be a base to create linux packages (deb/rpm) or a nsis script for windows in the future.
Sorry, i always forget MSVC doesn't handle simple #elifs well. An alternative is:
```
#elif defined(Q_OS_WIN)
```
Adding the defined() macro around the define should make MSVC an happy puppy.