[FIX] ownomogram: Fix wrapped C++ obj error

[irgolic]

Issue

#4998 was throwing this

Traceback (most recent call last):
  File "/home/runner/work/orange3/orange3/.tox/coverage/lib/python3.7/site-packages/Orange/widgets/visualize/ownomogram.py", line 266, in brushColor
    item.setBrush(value)
RuntimeError: wrapped C/C++ object of type DiscreteMovableDotItem has been deleted

This PR fixes it.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5005 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5005   +/-   ##
=======================================
  Coverage   84.65%   84.65%           
=======================================
  Files         284      284           
  Lines       58935    58943    +8     
=======================================
+ Hits        49891    49900    +9     
+ Misses       9044     9043    -1