Make migration assistants non-modal
If the migration assistants are non-modal:
- multiple migrations can happen concurrently
- existing migration won't block the user's interaction if it takes longer time to finish them
- eventually migrations can be invoked in the background (also by scripts)
Challenges:
- keep eye on various types of conflicts when multiple migrations are invoked
- progress reporting and interaction with the user when multiple migrations are invoked
- progress reporting can be displayed in extra common view as well