Concatenate widget: no union of attributes

[markotoplak]

Orange version

At least 3.3.8+, also affects master

Expected behavior

Given two different data sets to Concatenate and chosen option to output union of attributes would make a new data set with domain containing attributes from both data sets.

Actual behavior

Only attributes from the primary data set are on the output.

[markotoplak]

Hmmm, looking at the widget text this seems like this was actually a feature.

[markotoplak]

So the widget works as follows: union (or intersection) of features only works when there are no Primary Data. If there are Primary data, its domain is always used for the concatenated data set. Which was at least for me not what I expected when I sloppily used this widget. To make this clearer I propose deactivating "Domain Merging" box when "Primary Data" is on input in #1886. I also changed default inputs to "Additional Data".

I think the best would be to rename the multiple input "Additional data" to "Data" and "Primary Data" to "Domain" (and only use the Domain of the data set connected to this input). Because this would break compatibility of saved schemas, I propose this change for a new feature release.