[i18n] mechanism to overwrite TEI Publisher common labels

For custom TEI Publisher-based apps ideal i18n customization and fallback scenario would involve TEI Publisher backend and local project backend:

• TEI Publisher backend (all tei-publisher labels and their community-maintained translations)
• Local project backend (for customized labels)
  • app custom app namespace for project-specific labels
  • common for overwriting tei-publisher labels

NB TEI Publisher v6.0.0 already implements double-backend scenario and custom project labels. Only overwriting tei-publisher labels is missing