Add dark theme and default theme switching

[IanButterworth]

Adds ProfileView.set_theme!(:dark) for a dark mode.

Some limitations:

• I tried to find something that worked with the distinguishable_colors based method of FlameGraphs but I think dark themes generally lean to darker, rather than max contrast. And the text isn't standing out well. Not sure how to improve that?
  Update: PR to fix this Add darkmode kwarg to FlameColors and default_colors_dark FlameGraphs.jl#51

• Auto-detecting the Gtk theme and whether it's a dark theme would be nice, but I couldn't figure that out. Plus, on Ubuntu the Gtk window observes the system theme, but on MacOS it doesn't.

Both of these are with the system in dark mode and ProfileView set to :dark

Ubuntu:

MacOS:

[codecov]

Codecov Report

Merging #193 (83d5a57) into master (7d4c8c4) will increase coverage by 0.07%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   79.92%   80.00%   +0.07%     
==========================================
  Files           2        2              
  Lines         274      275       +1     
==========================================
+ Hits          219      220       +1     
  Misses         55       55              

Impacted Files	Coverage Δ
src/ProfileView.jl	85.93% <92.30%> (+0.05%)	⬆️

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