Multilingual setup

[janezd]

Orange 3.38 might support switching between languages. This is a draft of how add-ons would support that.

We would upload original sources to PyPi and run translation during installation. Add-on is "translated" if (see here)

• setup is run as install (not develop),
• Orange version is at least 3.38,
• trubar is installed. I propose we'd make it a requirement although Orange doesn't depend on it -- only translation of add-ons does. (Note: the current release of Trubar supports only translation to a single language, not switching. This PR suppose a new version which is still in development.)

If add-on supports multiple languages but Orange doesn't, or vice-versa, everything should work.

pip install -e . or python setup.py develop should keep source intact. Hence developers wouldn't (have to) notice that add-ons become multilingual when installed. Orange itself would also only be translated when preparing the release, not earlier.

Note: this only works if add-on is installed via python setup.py install or pip install --no-build-isolation .. Without --no-build-isolation, pip creates some kind of temporary virtual environment that has nothing but built-in Python modules.

@markotoplak, thanks for bringing setup.py to my attention. What do you think about this?

To do:

• [ ] How does this interact with CI?
• [ ] Can we move InstallMultilingualCommand to another place (where?!?!) where it can be shared between add-ons?