py2app script to create Mac application bundle

This is an improved version of the py2app script used in MacPorts, and is not dependent on that specific package manager anymore.

This version of the script will be used in MacPorts with the next release of Frescobaldi.
It should be the preferred method to create a Mac application bundle, also with other Mac package managers (such as Homebrew) and in manual installations.

At the moment the application bundle is NOT self-contained, so it is not distributable as a precompiled package.