[ENH] Sentiment Analysis - Language from corpus

[PrimozGodec]

Derived from #874

Description of changes

Use language from the corpus for Sentiment analysis

This PR also fix some other problems:

• Widget run computation in a separate thread to prevent blocking of widget
• Offline errors had to be re-implemented (widget does not check available languages on start anymore) since it can trigger the download of models which block the widget. Now models are downloaded before sentiment is computed in the separate thread.

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #954 (2531c33) into master (9bd6012) will increase coverage by 0.20%.
The diff coverage is 92.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #954      +/-   ##
==========================================
+ Coverage   77.79%   78.00%   +0.20%     
==========================================
  Files          87       87              
  Lines       12302    12282      -20     
  Branches     1613     1609       -4     
==========================================
+ Hits         9570     9580      +10     
+ Misses       2433     2413      -20     
+ Partials      299      289      -10