MacOS compatibilitly for the Converter Tool

[ChristopherRemde]

Currently, the Converter tool is only builded for, and only runs on Windows.

#10 makes it possible to run the Converter on MacOS, but without texture conversion support. We'd need to swap the existing windows-only texture conversion libraries with platform-independent ones.

Right now, the Converter Tool also can't be built because of: pyinstaller/pyinstaller#9335
When this issue is fixed, a MacOS build can be done with:

pyinstaller Sequence_Converter_UI.py --collect-all=pymeshlab --collect-all=numpy --icon=resources/logo.icns --windowed

[rokm]

The hook for pymeshlab will be added in pyinstaller/pyinstaller-hooks-contrib#975, and should alleviate the need for --collect-all=pymeshlab on any platform (and should avoid triggering pyinstaller/pyinstaller#9335).

And --collect-all=numpy should have been redundant all along - if not, it is either an issue with your python environment or a bug in PyInstaller's numpy hook that we should know about.