[FIX] Minor improvements to pythagorean trees

[pavlin-policar]

Description of changes

This fixes a couple of minor things I came across while reviewing #3775.

1. Remember selection in the Pythagorean forest widget so that reopening a workflow won't clear selection.
2. Remove some dead code from both widgets
3. Properly use context settings in Pythagorean tree and forest widget (before, it wasn't using the domain context settings handler.
4. The tree widget would crash if there was no data and we clicked "Redraw".

The tree widget does not currently remember selection (both when reopening a workflow or when redrawing), and this will be a bit trickier to implement, since Orange trees have no identifier for each node.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3777 into master will increase coverage by 0.01%.
The diff coverage is 87.3%.

@@            Coverage Diff             @@
##           master    #3777      +/-   ##
==========================================
+ Coverage   85.04%   85.06%   +0.01%     
==========================================
  Files         374      374              
  Lines       69111    69142      +31     
==========================================
+ Hits        58776    58816      +40     
+ Misses      10335    10326       -9

[thocevar]

There is a problem with the settings of Pythagorean trees that come from a forest. For example, you can set the depth of a tree to 1, then select a different tree and the visualization does not match the depth setting. Similar situation arises when you change the data. The depth is set to the context-aware value but is not reflected in the visualization.

[pavlin-policar]

@thocevar This issue should now be resolved.