Split: whitespace and empty space

[janezd]

Issue

1. Comma-separated text usually includes a space after comma. When widgets splits by comma, we then have two attributes - those starting with space and those without. Whitespace at the beginning and end is not shown, hence having it will never be intentional, imho. Thus it needs to be removed.
2. If empty strings are considered missing then in case the string is empty, values of all new variables should be shown as missing. Current implementation instead adds an indicator variable for missing strings, which is probably not even used, (r used in case of two consecutive delimiters?)
3. However, instead of fixing 2, I'd prefer to interpret empty strings differently in this context: they are empty because they do not contain any of the values into which the string is split. I think this makes more sense.

Description of changes

Fix 1, fix 2 via 3.

Includes

• Code changes
• Tests

[janezd]

bar with leading space must be the same as bar without.

[codecov-commenter]

Codecov Report

Base: 40.42% // Head: 40.42% // No change to project coverage 👍

Coverage data is based on head (f839f3c) compared to base (6b5c959).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #256   +/-   ##
=======================================
  Coverage   40.42%   40.42%           
=======================================
  Files           7        7           
  Lines        1269     1269           
=======================================
  Hits          513      513           
  Misses        756      756           

Impacted Files	Coverage Δ
orangecontrib/prototypes/widgets/owsplit.py	95.74% <100.00%> (ø)

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[janezd]

Now I'm scared. Seriously. I already fixed this in (still to be merged) #253. Now that I see it, I remember.