Migrate to pyenv Python Platform; Fix MacOS package (Major change)

The major change to the packaging script is meant to repair and improve the packaging mechanism for MacOS. The MacOS package was broken due to the new dependencies of the Qt libraries and the migration to pyenv platform.

The packaging script, devtools/deploy/mac/mac_package.py, is fully re-written in Python for clarity, as shell scripting was not suitable for such a complicated task. In essence, the script follows the rules explained in man dyld (1). Furthermore, it uses MacOS native tools otool and install_name_tool to adjust the binaries.

The script finds the dependencies of the core libraries up to a maximum level. The dependencies are then copied to the corresponding package directories, and library references and RPATHs are modified accordingly.

The Python platform for building will be pyenv (on Windows: pyenv-win).