Data sampler: Allow selecting 100% of data, shuffle it

[janezd]

Issue

Closes #2035.

Description of changes

#2035 required:

1. Modified DataSampler that allows selecting all instances (not just n-1)
2. DataSampler doc mentioning that the widget shuffles the dataset
3. A test that checks that DataSampler really shuffles the dataset
4. An example in Test & Score docs showcasing how this can be done (like #3245)

I've done them all, but I didn't have a good use case in 4. @ajdapretnar, in an already agitated state because of certain issues around #3722, told me in no uncertain terms to remove it. I'm married for 17 years, so I know better than to complain. On the contrary, I must say that I didn't like that text either, so I was very happy to remove it.

Documentation still explains that choosing the entire data set shuffles it, which, I must say, should suffice.

(Joke aside, it indeed does.)

Includes

• Code changes
• Tests
• Documentation

[astaric]

Removed requirement 4 from the comment, as I do not remember anymore what the intended use case was.

[codecov]

Codecov Report

Merging #3727 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3727      +/-   ##
==========================================
+ Coverage   84.53%   84.54%   +<.01%     
==========================================
  Files         373      373              
  Lines       68472    68490      +18     
==========================================
+ Hits        57886    57902      +16     
- Misses      10586    10588       +2