[osx] - replace alias shortcut resolving with non-deprecated API

We were using an outdated API for resolving osx alias / shortcuts. This PR uses the currently suggested API.

This fixes a regression we hit on OSX 10.10 yosemite. The API used in this PR is save down to 10.6 which is our lowest supported OSX Version.

It was confirmed working by @Karlson2k and the user who reported the issue. I will do another sanity check on osx 10.9.

I consider this save for helix because its a path which isn't hit very often and the API is pretty clear.

I also finally implemented the unit tests for this (which were there but only with a comment "TODO implement unit test") :D - the test run already on jenkins which means the shortcut resolving worked on osx 10.8 (this is what the jenkins slave runs).

Up to the RMs @MartijnKaijser && @topfs2 
It's better to use it for all platforms, i.e. move it before `#if defined(TARGET_DARWIN_OSX)`