Delete build/pkg before building.
Make build files python3-ready.
Update osx installer scripts.
Add osx/themes directory with gtk2 themes.
Add sandbox entitlements.
Do not access themes dirs not allowed by sandbox.
Disable sandbox for now, as it interferes with passing args to FAHViewer. Probably need app groups entitlement and drop an args.json file in group container.
Add entitlements.plist to app bundle for use in codesign later.
Add .app extension to viewer app name used with 'open' command.
It didn't use to matter, but on my machine "open -a FAHViewer" fails, while "open -a FAHViewer.app" works.
Thev 'open' man page examples use the extension.
