[ENH] Edit Domain: Merge categorical values

[ales-erjavec]

Issue

Implements gh-3371

Also fixes and assertion error when two values are renamed to the same value.

Traceback (most recent call last):
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/Orange/widgets/data/oweditdomain.py", line 1165, in commit
    var = apply_transform(v, tr)
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/functools.py", line 803, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/Orange/widgets/data/oweditdomain.py", line 1430, in apply_transform_discete
    dest_codes = positions(dest_values)
  File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/Orange/widgets/data/oweditdomain.py", line 1427, in positions
    assert len(rval) == len(values)
AssertionError

Description of changes

Implement merging of categorical values.

Merging can be achieved by simply renaming individual values to the same name, or by selecting multiple values and pressing the merge button (shortcut ctrl + =) and entering/selecting a name in an editable combo box.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3725 into master will increase coverage by 0.27%.
The diff coverage is 96.48%.

@@            Coverage Diff             @@
##           master    #3725      +/-   ##
==========================================
+ Coverage   84.53%   84.81%   +0.27%     
==========================================
  Files         373      374       +1     
  Lines       68471    68749     +278     
==========================================
+ Hits        57884    58307     +423     
+ Misses      10587    10442     -145

[janezd]

I'd avoid importing functions from widget modules if possible because it messes up dependencies and complicates future refactorings. I think we'd better treat all functions in widget modules as private (even if not named as such), just as we treat widget classes as final.

Do we have a better place for functions like unique?

Perhaps better, isn't unique(s) same as iter(dict.fromkeys(s)) (formally from Python 3.7, de facto already in 3.6). In below code, it's used as list(unique(s)), which is, I suppose, the same as list(dict.fromkeys(s)).