Concatenating two corpora forgets text feature

[ajdapretnar]

Describe the bug
When concatenating two corpora with the same column set as text feature (say, "content"), Concatenate forgets the variable was set as the text variable.

To Reproduce
Steps to reproduce the behavior:

1. Take two corpora (i.e. Parlamint), set non-first string variable as text feature for both.
2. Concatenate.
3. Observe the new text feature. It is back to the default.

Proposed solution
Text features is currently only a property of Corpus (if I am not mistaken). Have the feature be encoded in data.attributes.