Split platform and features

[sdruskat]

Platform and features (e.g., editors) should be handled separately.

• Version platform and features separately
• Provide different products, e.g.,
  • Platform product (includes just the platform, users install features interactively as needed)
  • All-product (contains all released features)
    • two or more strands per product, e.g., stable, review, experimental
• Provide marketplace for "official" features (?) or provide update site for "official" features
• Different repos for different features