Added Mac-specific settings dialog that looks more native

This doesn't make the Settings dialog look perfect on Mac OS X, but it definitely makes it a bit more native.

Currently the patch uses an external module that I pushed to github.com/shadone/qtmacextras - it currently is BSD licenced but I am fine with re-licencing it to whatever suits you. If you prefer not to use submodules, I am also fine with copying the needed files into mirall repo.