Create a standalone macOS .app using PyInstaller

It looks like Brew does not accept GUI apps, so #9246 (closed) is no longer a viable option.

The Brew folks however did point me to PyInstaller, which seems to be a better option. It allow us to create a standalone .app which we can distribute ourselves and optionally submit to Brew Cask (their repository for binary/GUI apps).

I managed to get a working .app with PyInstaller, but I haven't extensively tested it yet (portability etc). You can try it out by installing PyInstaller (pip3 install pyinstaller), cd to the gajim repository root, download gajim.spec and run:

pyinstaller gajim.spec
# and to package it in a dmg (optional):
rm -rf dist/launch && hdiutil create -volname Gajim -srcfolder dist -ov -format UDZO gajim-1.1.3.dmg

I did run into PyInstaller#4064. I'm not sure what exactly I did to work around it.. I believe I installed a few different versions of virtualenv and distutils and then it suddenly worked. Issue was fixed but unreleased as of v3.5)