Split: Move widget from Prototypes

[janezd]

Issue

Widget Split is in prototypes, but it's mature and useful.

Description of changes

Move it here.

Don't forget to merge biolab/orange3-prototypes#285 after merging this.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

❌ Patch coverage is 96.71053% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.80%. Comparing base (5eb97b8) to head (559b5af).
⚠️ Report is 163 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7153      +/-   ##
==========================================
+ Coverage   88.77%   88.80%   +0.02%     
==========================================
  Files         334      335       +1     
  Lines       73671    73857     +186     
==========================================
+ Hits        65402    65586     +184     
- Misses       8269     8271       +2     

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[janezd]

@ajdapretnar, would you be so kind to write the documentation? If you have time, of course.

[ajdapretnar]

One does not have time for documentation, one takes time for documentation.

[ajdapretnar]

Documentation PR available at: biolab/orange3-doc-visual-programming#12