Re #6563. Add library that exists in newer PyQt to installer.

In newer PyQt versions (e.g. 4.9.6), there's an additional library that
appears to contain most of the symbols, with the QtCore.so etc
libraries just being thin wrappers that import it. This library needs
to make it into the install kit if it exists.
Merge branch 'feature/6563_osx_dragndrop_installer' into develop