OWScatterplotGraph: Use get_color_labels to return number formatter

[janezd]

Issue

Fixes #3746 - displaying of datetime variables in visualization legends, as well as, probably, weird number of decimals for other numeric attributes.

Description of changes

Labels for color legend are created by palette, which is not aware of attributes. Palette is called by OWScatterplotGraph, which also doesn't know about attributes. On the other hand, the widget, which provides labels for discrete attributes (via get_color_labels), doesn't know how the graph will discretize continuous values, hence it can't provide a list.

A reasonable solution is that get_color_labels, which currently returns None for continuous variables, returns a formatter, that is, function that takes a number and returns a string. For most cases, this will be variables str_val. If it returns None, the widget uses the same formatter as it did before (a certain number of decimals, computed based on God knows what),

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3847 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3847      +/-   ##
==========================================
+ Coverage   84.94%   84.94%   +<.01%     
==========================================
  Files         376      376              
  Lines       70157    70164       +7     
==========================================
+ Hits        59595    59601       +6     
- Misses      10562    10563       +1