[ENH] Outliers: Offload work onto separate thread

[VesnaT]

Issue

Responsive Outliers widget: the widget could make calculations in a separate thread to make Orange responsive.

Description of changes

• Create a progress_callback function that serves as a wrapper function for callbacks. It allocates the corresponding proportion of the progress to the callback.
• Add callback to Learner to enable preprocessing interruption.
• Add callback to _OutlierModel.
• Do work in a separate thread.
• Still impossible to terminate the fit/predict operations.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4412 into master will decrease coverage by <.01%.
The diff coverage is 91.52%.

@@            Coverage Diff             @@
##           master    #4412      +/-   ##
==========================================
- Coverage   87.46%   87.45%   -0.01%     
==========================================
  Files         405      405              
  Lines       73968    74118     +150     
==========================================
+ Hits        64693    64817     +124     
- Misses       9275     9301      +26

[markotoplak]

Thank you @VesnaT, I like it a lot!

[janezd]

I glanced over this PR. The only thing I don't like is the name of the argument. callback can be anything. Perhaps progress_callback? Longer, but more informative.