[FIX] VizRankDialog: Stop computation when parent widget is deleted

[VesnaT]

Issue

Description of changes

Includes

• Code changes
• Tests
• Documentation

[astaric]

@janezd what about widgets emitting the signals and vizrank subscribing to them? Is there a special reason why vizrank did not use that in the first place?

[janezd]

It would work almost exactly the same, except that we'd have to define a signal, emit it and then write a method that receives it ... and finally sets exactly the same flag that is now set directly by the widget.

The problem here is not the communication but synchronization, that is, what happens if the widget is torn down before vizrank reacts, so vizrank tries to communicate with a widget that Qt already killed, and so forth. Signals wouldn't solve this.

[astaric]

The change crashes orange on my computer. The problem is in vizrank.deleteLater(), which deletes the widget before the computation continues, so the last step of computation (selecting the first row an removing progress percentage from caption) fails. If I remove the offending line, everything seems to work.

[codecov-io]

Codecov Report

Merging #2118 into master will decrease coverage by 0.96%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #2118      +/-   ##
==========================================
- Coverage   71.49%   70.52%   -0.97%     
==========================================
  Files         318      318              
  Lines       54423    54424       +1     
==========================================
- Hits        38908    38381     -527     
- Misses      15515    16043     +528

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