Added settingsAboutToBePacked signal to OWWidget

[markotoplak]

Issue

storeSpecificSettings is only called with a ContextHandler. Also, it is called after packing normal settings and before packing context settings.

In #3529 I implemented a special SettingsHandler because I needed to prepare settings for saving just before saving them.

Then I tried, in #3541, to extend (store|retrieve)SpecificSettings to an ordinary SettingsHandler. I failed: they were designed and used in a way that I could not make them work with ordinary settings (and not break stuff).

Before, I thought making storeSpecificSettings would be easy and would not change the interface. But now the question is if something like this is useful enough to extend the OWWidget with. I only needed something like this twice.

Description of changes

Added a signal settingsAboutToBePacked.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3567 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3567      +/-   ##
==========================================
- Coverage   83.97%   83.97%   -0.01%     
==========================================
  Files         370      370              
  Lines       66941    66969      +28     
==========================================
+ Hits        56215    56237      +22     
- Misses      10726    10732       +6

[codecov]

Codecov Report

Merging #3567 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3567      +/-   ##
==========================================
- Coverage   83.97%   83.97%   -0.01%     
==========================================
  Files         370      370              
  Lines       66941    66969      +28     
==========================================
+ Hits        56215    56237      +22     
- Misses      10726    10732       +6