Allow adding book content at runtime

[klikli-dev]

• Categories, Entries and Pages
• Handle sync, book state, etc
• No update of the ground truth data files -> restart/reload is expected to re-register the additional content
• Handle extension point well -> on data reload the additional content registration must be called again, or the additional content must be cached at reload and inserted again automatically.
  • called again at reload is better -> dynamic content may change e.g. based on available tags and recipes