gr: support horizontal legends

[t-bltg]

Fix #2206.
Fix #4497 (and add regression tests).

It's a tough one since the gr code is a mess, non generic, easily breakable (some legend code paths are not tested in ref imgs), etc...

Add the ability to draw legend bounding boxes (for debugging, when using Plots.debugplots(true)).
Fix centering of legend boxes (:top and :bottom), and position of legend title.

These examples should cover most of the legend boxes possibilities:

julia> using Plots; Plots.debugplots(true);
julia> kw = (legend_column=-1, legend_title="title");
julia> plot([1:2 reverse(1:2)]; marker=:circle, kw...)
julia> plot(1:2, fillrange=reverse(1:2); kw...)
julia> plot(rand(100); reg = true, fill = (0, :green), kw...)
julia> hspan([1, 2, 3, 4]; kw...)

Supersedes #4444.

Xref #2423.

Added new examples to cover the issues encountered in #4485 and #4491:
1.35.8 - inner - ex64

pr - inner - ex64

1.35.8 - outer - ex65

pr - outer - ex65

[codecov]

Codecov Report

Base: 90.88% // Head: 90.64% // Decreases project coverage by -0.24% ⚠️

Coverage data is based on head (e66a017) compared to base (9a8e9c7).
Patch coverage: 80.99% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4496      +/-   ##
==========================================
- Coverage   90.88%   90.64%   -0.25%     
==========================================
  Files          40       40              
  Lines        7650     7354     -296     
==========================================
- Hits         6953     6666     -287     
+ Misses        697      688       -9     

Impacted Files	Coverage Δ
src/backends/gr.jl	92.15% <76.76%> (-0.45%)	⬇️
src/examples.jl	98.50% <100.00%> (+0.17%)	⬆️
src/layouts.jl	95.49% <100.00%> (-0.24%)	⬇️
src/utils.jl	93.23% <100.00%> (+0.08%)	⬆️
src/Plots.jl	78.94% <0.00%> (-5.27%)	⬇️
RecipesPipeline/src/api.jl	81.81% <0.00%> (-1.52%)	⬇️
src/backends.jl	93.68% <0.00%> (-1.17%)	⬇️
src/backends/hdf5.jl	83.58% <0.00%> (-0.93%)	⬇️
RecipesPipeline/src/series_recipe.jl	92.30% <0.00%> (-0.80%)	⬇️
RecipesPipeline/src/group.jl	94.02% <0.00%> (-0.57%)	⬇️
... and 24 more

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[t-bltg]

@BeastyBlacksmith, I've updated all the images in this PR, and I think we have the correct behavior in terms of positioning and symmetry.

Do you have any more comments, before this goes in (pending ci) ?

I am going to make it Plots@1.36.0 after merge, since there are some slight box positioning changes / fixes in vertical mode.

[BeastyBlacksmith]

Thank you for working on this :)