Dark mode support

[fespinoza]

I'm currently preparing for dark mode:

Changes

• Add semantic colors for the default concept colors within the app
• Add a preferences window where one can override the app interface appearance (wanting to use the app in light mode while the rest of the system is dark mode for example)
• As for now, I'm just changing the content color of concepts that are fully black so in dark mode they have a white text color instead
• Drop support for macOS older than Mojave
• Windows auto save their frame data

TODO

• [x] clean up code
• [x] group selection color
• [x] concept selection color
• [x] link creation color
• [x] link selection color
• [x] persist setting of preferred theme