macOS fixes/improvements
#1445 (closed) is already tracking the build failures with Xcode 10, but there are other improvements for macOS which I'd like to work on:
- a QuickLook plugin so graphs could be previewed from the Finder, Mail, etc.
- it would be really cool to make a self-contained Graphviz.app, so viewer-only users wouldn't need to bother with a .pkg or Homebrew. Maybe even put it in the Mac App Store if possible.
- If a self-contained app is possible, maybe a .pkg is unnecessary?