Re #6564. Remove items applicable to older OS X.

i.e. versions that we no longer support.
Re #6563. Remove installer preflight script.

It was just cleaning up long-gone items so I don't think we
need it anymore. It can't be used with a DragNDrop installer anyway.
Re #6563. Switch to drag and drop install kit.

Replacing PackageMaker.
Re #6563. DragNDrop generator need plain text license only

It can't do anything with the rich text files.
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 remote-tracking branch 'origin/feature/6563_osx_dragndrop_installer'