enemy: allow non-harmful enemies

[jorgegv]

Allow some enemies that move around exactly as real enemies but do not kill or harm the hero (i.e. decorative moving sprites)

Tasks:

• [ ] Add a flag to the enemy data struct: F_ENEMY_IS_HARMFUL
• [ ] DATAGEN: add support for setting initial state for an enemy, default HARMFUL
• [ ] collision: only do harm to the hero if F_ENEMY_IS_HARMFUL is set
• [ ] FLOW: add functions to set/reset an enemy flag