deprecate_panel_opacity

[Mattriks]

Deprecates Theme panel_opacity. See GiovineItalia/Compose.jl#322.

• I've added a dep string to @Varset Theme
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

[codecov-io]

Codecov Report

Merging #1219 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1219   +/-   ##
=======================================
  Coverage   93.66%   93.66%           
=======================================
  Files          33       33           
  Lines        3156     3156           
=======================================
  Hits         2956     2956           
  Misses        200      200

Impacted Files	Coverage Δ
src/theme.jl	86.84% <ø> (ø)	⬆️
src/guide.jl	87.06% <ø> (ø)	⬆️

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