dmgbuild module is broken with Qt Creator hosted builds of Qbs

The dmgbuild module is not working in the version of Qbs packaged with Qt Creator 4.4. Specifically, the dmgbuild Python executable is not deployed into the Qt Creator application bundle, and the dmgbuild module would look in the wrong directory for the module anyways.

Users can work around the issue for now by copying the dmgbuild executable from src/3rdparty/python/bin/dmgbuild in the Qbs source directory to the "Resources" directory of the Qt Creator app bundle, and setting the dmg.libexecPath property to that location. For example:

dmg.libexecPath: "/Users/$USER/Qt/Qt Creator.app/Contents/Resources"