[FIX] owimpute: Make default_numeric locale independant

[ales-erjavec]

Issue

The default_numeric parameter in OWImpute is stored as localized string and is hence not portable.

Steps to reproduce on macOS:

• Open System Preferences.app -> Language & Region -> Advanced..., Change decimal separator to '.'
• Open Orange, add File (load say brown-selected) -> Impute -> Table. In Impute set default fixed numeric value to '1.222', Observe that on the output the missing values are correctly replaced.
• Save the workflow and close Orange
• Open System Preferences.app -> Language & Region -> Advanced..., Change decimal separator to ','
• Open Orange and load the saved workflow
• Observe that in the Impute widget the numeric fixed value text is '1.222', and that on the output the missing values remain missing.

Description of changes

Replace use of QLineEdit with QDoubleSpin box for editing. Unify min/max/precision in default method and individual variable spin boxes.

No migrations since the change that introduced the default fixed values was not yet released.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5209 (703320c) into master (0d6da92) will increase coverage by 0.01%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##           master    #5209      +/-   ##
==========================================
+ Coverage   85.17%   85.19%   +0.01%     
==========================================
  Files         299      300       +1     
  Lines       60888    60947      +59     
==========================================
+ Hits        51864    51924      +60     
+ Misses       9024     9023       -1