macOS: Setup .app bundle build for Raven

[timlehr]

Currently Raven doesn't build an .app bundle for macOS. The user experience would improve a lot, since macOS users are accustomed to using .app for their work. Even better: Drag & dropping a file onto the App bundle could open Raven with that file.

[timlehr]

I took a shot at this a while ago for an internal build, but it's more of a proof on concept and needs a lot more polish. Also: No drag & drop. If somebody wants to pick this up, please do. #18