[FIX] OWImpute: Preserve default method setting

[ales-erjavec]

Issue

• The default method changes to 'Fixed values' when the widget receives first data.
  Steps:
  * create an 'Impute' widget - do not connect it yet
  * change the default method to say 'Average/Most frequent'
  * connect it to 'File', the method changes to 'Fixed values'

• The date time setting is reset at widget creation and is not restored.

Description of changes

• Preserve the default method on set_data
• Do not reset default_time in init
• Do not switch to 'Fixed values' by tabbing over controls
• Fix type hint comment location

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5181 (b69a634) into master (1d2626e) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #5181      +/-   ##
==========================================
- Coverage   84.79%   84.79%   -0.01%     
==========================================
  Files         286      286              
  Lines       60189    60192       +3     
==========================================
+ Hits        51036    51038       +2     
- Misses       9153     9154       +1