PGFPlotsX: reworks

[t-bltg]

• fix Unicode error with PGFPlotsX [BUG]  #3219;

• fix [BUG] LaTeX error in PGFPlotsX backend. #4182;

• rework aspect_ratio after forward aspect ratio for 3D plots in pgfplotsx #4231 (comment);

• lots of code simplifications (consistency, modernization) - remove ~140 lines.

• build docs => no apparent regression

Needs korsbo/Latexify.jl#223.

[codecov]

Codecov Report

Merging #4262 (6bd4db4) into master (1cafdb0) will increase coverage by 0.24%.
The diff coverage is 71.23%.

@@            Coverage Diff             @@
##           master    #4262      +/-   ##
==========================================
+ Coverage   79.67%   79.91%   +0.24%     
==========================================
  Files          29       29              
  Lines        7052     6962      -90     
==========================================
- Hits         5619     5564      -55     
+ Misses       1433     1398      -35     

Impacted Files	Coverage Δ
src/backends/pgfplotsx.jl	71.84% <70.96%> (+1.24%)	⬆️
src/axes.jl	86.43% <100.00%> (+0.43%)	⬆️
src/colorbars.jl	87.50% <100.00%> (-0.20%)	⬇️

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[BeastyBlacksmith]

can you split the functional and the aestetics part of this PR into two PRs?

[t-bltg]

can you split the functional and the aestetics part of this PR into two PRs?

I've rebased the whole PR into multiple commits, so it can be more easily reviewed.

I've added more tests for LaTeXStrings and Latexify.

I've also fixed the aspect_ratio after #4231 (comment).

[BeastyBlacksmith]

I am reckognizing that code format is a highly opinionated topic, but some changes here (that I marked with ^) are reducing readability and maintainability in favor of terseness.

In general I try to write code in a way that there is mostly happening one thing at a time on one line and that you have to change few things if you need to extend or refactor the code.

[t-bltg]

I am reckognizing that code format is a highly opinionated topic, but some changes here (that I marked with ^) are reducing readability and maintainability in favor of terseness.

Thanks for the review. I've tried to answer all the comments, explained why some are IMO justified and rebased the PR to remove the modifications on which we agreed are unjustified or not needed.

[t-bltg]

I am going to merge the PR when CI passes, unless something new is brought to the table.