Support Dark Mode

[allenmonroesmith]

Dark mode should work.

[undecoded-coder]

Minor known issues

1. Line/conditional line icons in outline are black
2. Placard button backgrounds are white

I have a basic fix for 1 right now (gray icons which can be seen in light or dark mode just fine). In order to support adaptive images I'm working on migrating assets to an asset catalog. After that the file outline view will need to be made view-based (instead of cell-based) in order to support adaptive images via NSImageView (currently just draws the image into the cell frame, which is not adaptive).
Once I'm done with the asset catalog I can look at the placard icons (2) as well.

[undecoded-coder]

New placard icons added in the original PR