Mosaic VizRank: Start when opened, pause when closed

[janezd]

Issue

Ref. #6504. I also strongly suspect

Based on #6614 (for pause_when_closed flag, and for CloseVizrankMixin). Only the last commit is new.

@processo was totally right in #6504. I knew that closing VizRank doesn't stop it, but I expected that closing the widget would. Of course it doesn't, and in case of Mosaic, which rapidly fills the table model and fires signals, it blocks the main thread, making Orange basically unresponsive even after the widget is closed and everything should have been silent.

When #6612 or alike is merged, we should prevent vizranks from sending such frequent updates.

Description of changes

Changes are similar to those made to vizranks in #6614.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6632 (5970064) into master (df20f2e) will decrease coverage by 0.02%.
Report is 2 commits behind head on master.
The diff coverage is 82.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6632      +/-   ##
==========================================
- Coverage   88.10%   88.08%   -0.02%     
==========================================
  Files         321      321              
  Lines       69876    69906      +30     
==========================================
+ Hits        61563    61578      +15     
- Misses       8313     8328      +15