Change Virtual env for CQ operation

Hi all,

Thanks for the great project

I have some projects which are starting to use CadQuery. These projects have their own dependency situations so each have a virtual environment. It would be really nice if we could select the python environment within CQ-editor rather than have to add a copy of CQ-editor to each environment.

It would also be good if this environment selector wasn't pinned to Conda, as I don't use Conda