[ENH] Computation in separate thread for base vectorizer; use base vectorizer for embedding

[PrimozGodec]

Issue

The original issue is that the Embedder widget and embedder method is not based on the base vectorizer.
To enable basing on BaseVectorizer/OWBaseVectorizer several changes need to be made in both classes

Description of changes

• Implement concurrent mixin in OWBaseVectorizer (consequently it is used in Simhash and Bow)
• Reimplement commit (do not compute if the commit is unchecked until the button pressed)
• Use deferred commit
• Change embedded to base on BaseVectorizer
• Add some tests

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #852 (76f9060) into master (4c38b3f) will increase coverage by 0.22%.
The diff coverage is 93.63%.

❗ Current head 76f9060 differs from pull request most recent head c378f76. Consider uploading reports for the commit c378f76 to get more accurate results

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
+ Coverage   76.67%   76.90%   +0.22%     
==========================================
  Files          83       83              
  Lines       11761    11772      +11     
  Branches     1867     1866       -1     
==========================================
+ Hits         9018     9053      +35     
+ Misses       2439     2415      -24     
  Partials      304      304              

[ajdapretnar]

One other issue. When saving an old workflow with Document Embedding, the workflow doesn't work well with the new version. The issue is the "Embedding" output is now renamed to "Corpus". Should probably be added to settings migration somehow.

[PrimozGodec]

One other issue. When saving an old workflow with Document Embedding, the workflow doesn't work well with the new version. The issue is the "Embedding" output is now renamed to "Corpus". Should probably be added to settings migration somehow.

It is fixed now through the replaces argument