Less cardinal specific changes

[falkTX]

Now that VCV Rack has an official dark theme switch we can get a bit closer to the official Rack approach.
The Cardinal custom rack::settings::darkMode is being deprecated in favour of the official rack::settings::preferDarkPanels

I still do not like having the "mid" as the initial surge theme though, so light/dark switch remains for the initial/first instance of your plugins.

Thanks for the nice modules once again

Next Cardinal release is planned for 15th of January btw.

[falkTX]

now that I see #921 it is then better to not merge this, because the integration with the new rack API is not completed.

I will force-push soon so that rack::settings::darkMode is updated, but stick with everything else as-is.

PS: if you want to have compat for new Rack APIs while being usable on old Rack versions I think a dlopen(null) + dlsym would do the trick, basically finding the location of the "bool preferDarkPanels" variable that lives within libRack.

[falkTX]

ok I redid the PR and the changes are very minimal now, it is okay for merging from my side.

[baconpaul]

Thanks!