Rework warn_on_unsupported

[t-bltg]

I need this to go in for extra_kwargs to work properly in UnicodePlots.
warn_on_unsupported should be evaluated per backend.

[codecov]

Codecov Report

Merging #4081 (6db248d) into master (824dbe9) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4081      +/-   ##
==========================================
- Coverage   62.22%   62.22%   -0.01%     
==========================================
  Files          29       29              
  Lines        7293     7295       +2     
==========================================
+ Hits         4538     4539       +1     
- Misses       2755     2756       +1     

Impacted Files	Coverage Δ
src/args.jl	72.45% <100.00%> (-0.12%)	⬇️
src/backends/unicodeplots.jl	53.37% <100.00%> (+0.31%)	⬆️

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[BeastyBlacksmith]

Why is that needed? Extra keyword arguments shouldn't be treated as unsupportet arguments. Or do you have keywords that conflict with Plots default keywords?

[t-bltg]

Because

Plots.jl/src/backends/unicodeplots.jl

Line 4 in 5facd2b

warn_on_unsupported_args(::UnicodePlotsBackend, plotattributes) = nothing

was a cheap way to remove unsupported arguments warnings, and basically disabled extra_kwargs by returning nothing.

[BeastyBlacksmith]

Can't we remove that line then?

[t-bltg]

Can't we remove that line then?

I already dit it in 3e8db45, replaced it with plt.attr[:warn_on_unsupported] = false, but that doesn't work (too late in the pipeline, and I didn't catch that because of the Revise workflow), and thus tons of warnings are appearing again.

[BeastyBlacksmith]

I see, makes sense