add empty! for Plots and Subplots

[t-bltg]

using BenchmarkTools, Plots

const mat = randn(512, 512)

f(x, n) = begin
  pl = plot()
  for _ in 1:n
      empty!(pl)  # clear out previous series
      heatmap!(x)
  end
  pl
end

@time display(f(mat, 100))  # warmup

@time display(f(mat, 100))
 1.255724 seconds (3.20 M allocations: 374.080 MiB, 19.67% gc time)  # PR
 32.482719 seconds (314.77 M allocations: 16.524 GiB, 8.18% gc time)  # master (without `empty!`, obviously)

[codecov]

Codecov Report

Base: 90.88% // Head: 90.89% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (384f4d7) compared to base (eb99990).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4543   +/-   ##
=======================================
  Coverage   90.88%   90.89%           
=======================================
  Files          40       40           
  Lines        7705     7707    +2     
=======================================
+ Hits         7003     7005    +2     
  Misses        702      702           

Impacted Files	Coverage Δ
src/types.jl	100.00% <100.00%> (ø)

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