Moving import and export to a worker

[TheLastProject]

Fixes #513

• [X] Move import/export to worker
• [ ] Notification permission (should work but needs more testing)
• [X] Show import state on main screen
• [X] Reload cards list after successful import
• [ ] Handle password protected imports again
• [ ] Fix unit tests to follow new flow
• [ ] Clean up all the no longer relevant string.xml files
• [ ] Other cleanups