[ENH] Orange.misc.environ config

[ales-erjavec]

Issue

Ref gh-4264

Description of changes

• Read configured paths from a .cfg file. The cfg file can use and define install prefix relative paths
• Use the defined paths to override the default paths for storing widget and canvas settings.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4576 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4576      +/-   ##
==========================================
+ Coverage   83.59%   83.60%   +0.01%     
==========================================
  Files         281      276       -5     
  Lines       56131    55406     -725     
==========================================
- Hits        46921    46323     -598     
+ Misses       9210     9083     -127     

[thocevar]

This will enable us to set the location of widget settings for a portable version
(#4264). I've managed to set up a config file and read/write settings to a specified relative path.