[ENH] Add Table Input to Corpus

[nikicc]

Issue

One cannot convert Orange's data table to Corpus instance.

Description of changes

• OWCorpus now has Table input. When present, some GUI elements are frozen and the data from the input is considered.
• Raise an error when corpus has not textual features.
• Raise an error when no text features are selected.
• CorpusViewer now accepts Corpus, not Table.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #308 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #308   +/-   ##
=======================================
  Coverage   85.04%   85.04%           
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  Files          34       34           
  Lines        1825     1825           
  Branches      333      333           
=======================================
  Hits         1552     1552           
  Misses        238      238           
  Partials       35       35

[lanzagar]

Context settings for lists of variables need to be handled correctly (in this widget and others) - planned for a separate PR.

[lanzagar]

Ah, merged it a little too quickly.
I overlooked that you are changing the input type for Corpus Viewer and don't insure backwards compatibility for saved workflows.