[FIX] gui.spinBox: Remove extraneous created widget box

[ales-erjavec]

Issue

Fixes biolab/orange3#6715

gui.spin creates an extra widget box on the parent which interferes with mouse interaction

I.e:

Note that the layout depends on the style, should be tested on Windows.

Description of changes

Remove extraneous created widget box

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (31e1b65) 71.72% compared to head (499df3a) 71.72%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   71.72%   71.72%           
=======================================
  Files          34       34           
  Lines        9086     9086           
=======================================
  Hits         6517     6517           
  Misses       2569     2569           

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