sprite: refactor and fix sprite code used for enemies in map screens

[jorgegv]

Nomenclature for enemies in screens is confusing: they are called "sprites" but they should be named "enemies". This makes code difficult to read and modify.

• Enemy management code in sprite.h and sprite.c should be separated into new enemy.h and enemy.c
• All code using those should be fixed (map.c, hero.c, etc.)
• DATAGEN new syntax for ENEMY lines in SCREEN sections
• DATAGEN should be fixed to generate the correct code with the new format
• DATAGEN documentation should updated
• GDATA files should be upgraded to the new syntax

[jorgegv]

merged in 9117c84