Add events for carried entities

Add events:

1. "carried_object:on_carried()", or even better, "carried_object:on_carried_by(other_entity)". This may allow some entities to escape from the hands of the hero (or an enemy) when he carries them, for instance.

2. "carried_object:on_thrown()", or even better, "carried_object:on_thrown_by(other_entity)". When this event is defined the "built-in thrown movement" should not happen. The devs of the game should code what happens then, including the falling trajectory. When the event is called, the entity is not being carried anymore.

3. "carried_object:on_fallen()", called when this entity has fallen to the ground after being thrown. This may be use to code entities that bounce on the ground, or NPCs/enemies that are restarted when thrown, and many other things (we can think more uses later).