[FIX] owimpute: Fix editing of individual imputers

[ales-erjavec]

Issue

Fixes gh-1920 and gh-1965

Description of changes

• Fix reseting of individual variable imputers back to default (Impute Widget: Cannot reset individual Attribute Settings to Default #1920)
• Prevent global modification of OWImpute.METHODS
• Properly disable the 'Value' {Combo,Spin}Box for setting fixed replacement values
• Fix editing of per-variable specific replacement values (Impute Widget: Individual 'Value' imputer editing does not work #1965)

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #1966 into master will not impact coverage.

@@           Coverage Diff           @@
##           master    #1966   +/-   ##
=======================================
  Coverage   89.53%   89.53%           
=======================================
  Files          90       90           
  Lines        9179     9179           
=======================================
  Hits         8218     8218           
  Misses        961      961

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