[ENH] Edit Domain: Option to merge less frequent values

[PrimozGodec]

Issue

Fixes #4436

Description of changes

Added an option to merge less frequent values:

• values that have less than n appearances
• keep only n different values

The current implementation of merging selected values moved in a new dialogue.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4477 into master will increase coverage by 0.28%.
The diff coverage is 98.00%.

@@            Coverage Diff             @@
##           master    #4477      +/-   ##
==========================================
+ Coverage   83.16%   83.45%   +0.28%     
==========================================
  Files         268      276       +8     
  Lines       53990    55228    +1238     
==========================================
+ Hits        44903    46091    +1188     
- Misses       9087     9137      +50     

[lanzagar]

After you select one of the merge options, set its parameter, apply the transformation and then see the result you might want to change the parameter. It is kind of annoying that opening the merge dialog again resets everything (it would be nicer to see what was selected previously and allow a minor correction).
Would that complicate the code a lot or is it simple to change?

[lanzagar]

Also, don't forget to put an [ENH] tag in the PR title (for the changelog).

[janezd]

This could be merged as is, but I have a list of small comments. Feel free to reject any or even all.

[janezd]

It is kind of annoying that opening the merge dialog again resets everything

I also thought about this. Then I considered print dialogs, which usually do not remember page intervals, for instance.

But I agree with @lanzagar - if it's not too complicated to include this as a setting, please do.

[PrimozGodec]

Now parameters are saved as a Context variable. The solution does not look nice, but I did not see any better option. If it looks too ugly we can remove it.

[lanzagar]

Apart from the extra sign in a docstring, the rest seems ok to me.