fix: btiles should be referenced by name, and their objects indexed

[jorgegv]

Several objects that appear in screens are drawn via BTiles (obstacles, decorations and items). THey normally have a NAME field which references the BTILE whith souhd be used to draw the object. THis blurs the distinction between the object and the tile, which is problematic because the same BTILE can be reused for several objects.

Objects and btiles should differentiated:

• Object definitions should have a NAME field and a BTILE field. The NAME should be used for referencing the object elsewhere (flow rules, etc.) and the BTILE should be used for drawing.
• Object types that should be migrated:
  • Obstacles
  • Decorations
  • Items

[jorgegv]

fixed in b517536