Support optional installation of pyobjc packages by dependency

[gselzer]

pip supports dependencies necessary on specific operating systems, written like this. I thought conda does as well, however at the very least we could install libraries optionally via pip within the conda environment file.

We should use these platform-specific dependencies to install pyobjc on mac, so that you can use gui mode right away.

[NicoKiaru]

Would that allow to use the interactive mode on mac or is this unrelated ?

[gselzer]

@NicoKiaru this would make it marginally easier to use gui mode from macOS (which is not quite the same as interactive mode).

[NicoKiaru]

Ah yes now I understand, I'm currently trying to use pyimagej on mac, and I had these dependencies missing:

• pyobjc-core
• pyobjc-framework-cocoa
  (edit: it's already in the docs)