Automatic language detection in Corpus with language listed in output

**Is your feature request related to a problem? Please describe.**
I have a data file with abstracts from PhD theses in a column "ABSTRACT". Most abstracts are in English, but some are in Italian, and I want to be able to filter by language (e.g. using Select Rows). I get the impression that the latest versions of the Text add-on (>= 1.13) support automatic language detection, and in #916 it is suggested that this has now been incorporated in Corpus. However, from what I see, the language still has to be selected by the user, and there is no column in the output that lists the language.

**Describe the solution you'd like**
In the Language drop-down menu, add the option "guess" to the list of languages, and add a column "Language" to the output that lists the language of each row.

**Describe alternatives you've considered**
As I've learned that appearance of the word "il" (="the") distinguishes Italian text from English text, I inserted a Feature Constructor with the following assignment `Language := 'IT' if 'il' in ABSTRACT.lower().split() or ABSTRACT.lower().startswith('il') else 'EN'
` but it works for this specific case only and it's rather awkward.

Start by locating the Corpus implementation and the Language drop-down handling in the Text add-on, then review issue #916 for the existing automatic detection work. Confirm how language values are represented before defining the output column and filtering behavior; done means a guess option detects each row's language and exposes it in Corpus output.

Written by the indexing model from the issue text.