Flint: Refactor the code with a viewers package

Closes #3663 (closed)

This refactoring aim to refactor the code in order to simplify the maintenance

• Creates bliss.flint.viewers package
• This package contains a package per viewers (live_curve, live_image...)
• Each viewer package is normalized as
  • viewer, the data viewer widget
  • property, the property editor for this widget
  • viewer_api, the flint side API served to BLISS
  • client, the code which is imported by BLISS
• Plus clean up of typing decoration taking opportunity of | operator and base types list/dict/set
• Plus fixes based on mypy reports