[RFC] Add-ons: towards fixing conda/pip issues

[markotoplak]

There are problems with installing add-ons into Orange installed with conda.

Anaconda, where a lot of users find Orange for the first time, comes with an out-of-date Orange version. Also, the default Anaconda channels do not include conda-forge. Thus, whenever users install add-ons that require a newer version of Orange, or, god forbid, use the add-on dialog to actually update Orange, Orange installs from pip and wreaks havoc on the current environment. Examples: biolab/orange3#5693 and problem when testing biolab/orange3#5064.

Possible improvements:

• Prevent pip installations into conda environment, except when explicitly allowed (perhaps a setting among the global settings).
• A dialog suggesting adding the conda-forge environment into available channels and that would add if the user agrees.
• Display (and install) versions as available on the current channels. After installation of the newest available version, perhaps show an indication that there exist newer versions on currently unavailable channels, and make some instructions on fixing that available.

@PrimozGodec already started preparing for this: he put most official add-ons onto conda-forge.

When the dialog is improved sufficiently, we could finally enable biolab/orange3#5064.

[markotoplak]

I am adding the patch from biolab/orange3#5064 here as a backup.

0001-main-Open-Installer-previously-Add-ons-dialog-fr.patch.txt