Python dependencies for widgets

[yxqd]

Widgets may have different dependencies. Do we need to add all dependencies to conda meta.yaml and setup.py? Some users of this package may only need one widget that has minimal dependencies, while some users may need widgets that requires installation of a lot of dependencies. Should we delegate to each widget to declare its dependency. Is the "extras_require" for setup.py good for this?

[JeanBilheux]

I will go with all-in-one. That won't be a huge package/library anyway and it really makes their life easier.