Warn if the settings in the workflow are from higher versions

[markotoplak]

If we open some newer workflow on old Orange, the results are undefined. It could work, it might crash, or it could load but not work properly (forgets some settings).

On loading settings we should confirm that the setting version of the available widget is higher or equal to the saved setting version. If not, we should warn the user, and (if possible) show what to update (core orange or a specific add-on).

What to do after the warning? I do not know. I suggest we still try to use the settings as they are. If it crashes, well, fine, there was a warning.

[markotoplak]

We are, as developers, more often that not running a newest version of Orange, so we do not see those problems that often. But our users are not, so we could easily underestimate the importance of this issue.

These warnings may be quite important.

Not everyone likes to update so often.
-- Someone running OS X 10.14, Ubuntu 18.04, and Android 4.4, who finally upgraded to Python 3.7 recently.

[janezd]

The check is trivial

• either orangewidget.widget.OWBaseWidget.migrate_settings and orangewidget.widget.OWBaseWidget.migrate_context throw an exception (to whom?) if the settings' version is too high,
• or orangewidget.settings.SettingHandler._migrate_settings and orangewidget.settings.SettingHandler._migrate_contexts compare versions (and do what?).

The former is more appropirate, the latter is safer, because current migration methods on widget never call the base class method. In either case, the latter two methods must then remove the settings/context, as if IncompatibleContext was raised.

I can do the above. But my knowledge of canvas is rather limited, and I don't know what would be a good way of communicating this to canvas, so it can show a dialog that some settings were removed.

With this, I'm unassigning myself, hoping that @ales-erjavec or @irgolic will take over.

[janezd]

This issue says we should check settings versions. On Slack we also discussed workflows. We could add checks for earlier and later versions and write something like:

• This workflow was created in Orange 3.26; you have 3.29. This is usually not a problem, but it cannot hurt to check that everything works.
• This workflow was created in Orange 3.29; you have 3.26. This usually fails. We recommend updating Orange.

In the latter case, the message appears only if there are no conflicting migrations discussed above.