Support Multiple Instances

[simonschuhmacher]

• It would be nice if multiple instances of the editor could be opened simultaneously
• Things we might have to take into consideration for this
  • Temporary files used for compilation of the presentation
  • Local storage

[triumphvalley]

When we do this, I would go for the whole package:

• Allow multiple windows at the same time
• Warn the user to save the content before closing any window
• Directly open new window when new file is selected
• Allow to drag a file onto the app icon to open it in a new window
• Menu item to open new window on app icon