Fix persisting colors on macOS [Fix #1564]

[karlingen]

We are not persisting the custom colors to REAPER.ini on macOS for some reason. The user has to restore the custom colors every time Reaper is restarted. This fixes #1564

It would be nice with an explanation as to why we weren't doing this in the first place and if I might have missed something vital.

[cfillion]

On macOS the custom colors should be saved into the user's global favorite color palette rather than the in the .ini. The root cause of the bug is SetCustomColors failing to save the colors to the system palette on high sierra and newer.

I wrote a potential fix, could you give 84621f5 a try?

[karlingen]

@cfillion Just gave it a go. It works! It also fixed the custom color picker 😃
Awesome work man!

Can we get a PR up for it? I'll close this one

[karlingen]

@cfillion Also for future reference, where are all of the NSFavoriteColorsStore and related APIs documented?

[cfillion]

Where are all of the NSFavoriteColorsStore and related APIs documented?

They aren't documented as far as I could find. Seems to be a private Apple API (could break again in future updates). I found those functions and classes by reverse-engineering how the system color picker saves the colors.