Store addons state as JSON

[SaschaCowley]

Link to issue number:

Closes #19559

Summary of the issue:

Addons state is saved in a pickle file.

Description of user facing changes:

None

Description of developer facing changes:

Description of development approach:

Switch to using lists instead of sets when populating AddonsState from dict.

Testing strategy:

Unit tests.

Known issues with pull request:

Code Review Checklist:

• Documentation:
  • Change log entry
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• API is compatible with existing add-ons.
• Security precautions taken.