Move from onefile to onedir in building for macOS

[machawk1]

onefile + macOS .app is deprecated and will become an error in PyInstaller 7; migrate to onedir.

13632 DEPRECATION: Onefile mode in combination with macOS .app bundles (windowed mode) don't make sense (a .app bundle can not be a single file) and clashes with macOS's security. Please migrate to onedir mode. This will become an error in v7.0.

[machawk1]

PR #603 might have also fixed #568. Opening the .app no longer causes it to close and re-open itself, which was always a weird behavior of pyinstaller-built apps.