Add game edition mini tutorial

[jrmi]

Add a read-only "game" that shows every aspect of game creation:

• Add an item
• Each item usage ?
• Layer system
• Basic item edition
  • Layer
  • Locking
  • Rotation/Size
  • Actions
• Multiple item edition
• Media handling
• Focus on some items
  • Image
  • Zone + auto action
  • Anchor
  • Generator
• Show some common actions
  • Create cards + Deck
  • Create dice
  • Create hands