Version 0.12.0

• Update to Bevy 0.15
• Add SequentialActions marker for required components
• Add skip amount in ModifyActions::skip
• Add multiple actions with a tuple
  • Removes add_many as add can now be used instead
• Add cleanup of actions for despawned agents
  • on_stop, on_remove and on_drop in the Action trait
    now take Option<Entity> for agent in order to respond to despawns
  • Replaces all unwraps with logging
  • Adds on_remove_hook and on_remove_trigger for both CurrentAction and ActionQueue