Installer/darwin: Clean up python build directory after installation, otherwise we'll end up breaking the bundle signature.

Installer/darwin: Can't switch between x86 and amd64 binaries with new signing (and we no longer need to).

Installer/darwin: 'ln -vh' => '/bin/ln -vh' -- see public ticket 12275

OS X installer: more checks for preinstall and postflight scripts.

OS X installer: keep previously installed extension packs on upgrade.

VirtualBox/postflight: python2.7

Installer-OSX: check that $USER is defined

FE/Qt4; Installer-OSX; Resources: add handling of file type association

Installer-OSX: install launchd description file for vboxwebsrv

Installer-OSX: change headers

export darwin installer to OSE