Improve window handling in Mac OS

[c3er]

The application currently only allows one window per process. From the user's perspective, that means that Markdown files opened in Finder will always open in the same window as the previously opened document.

Expected behavior:

• Files that are opened in Finder shall always open in a new window
• All windows shall open in the same process
  • The dock has to only show one icon for the application
  • This is actually an optimization that would also be a benefit in other OSs