gameworld.clear_entities()
        system = gameworld.system_manager['position']
        ent = gameworld.init_entity({'rotate':0}, ['rotate'])
        system.create_component(ent, 'general', (0,0))
        self.gameworld.remove_entity(ent)
        assert system.get_active_component_count() == 0

        gameworld.clear_entities()
        system = gameworld.system_manager['position']
        ent = gameworld.init_entity({'rotate':0, 'position':(0,0)}, ['rotate','position'])
        cind = gameworld.entities[ent].get_component_index('position')
        system.remove_component(cind)
        self.gameworld.remove_entity(ent)

    gameworld.clear_entities()
    system = gameworld.system_manager['position']
    ent = gameworld.init_entity({'rotate':0, 'position':(0,0)}, ['rotate','position'])
    system.create_component(ent, 'general', (0,0))
    self.gameworld.remove_entity(ent)
    assert system.get_active_component_count() == 0

This PR fixes/prevents all the mentioned cases.

**Changes:**
- Changed the `EntityManager.set_component` method to call `Entity.set_component` so the `load_order` can be updated. Not super happy about this but i think its the best backwards compatible way to do this.
- The `Entity.set_component` function checks if the system is already in the `load_order` and otherwise adds it. This makes component initialization a little bit slower because we need to search in the `load_order` list. This could be optimized, but according to my benchmarks it doesn't really matter.
- The `Entity.set_component` function will now raise a `DuplicateComponentError` if there is already a component from the system active.
- The Entities `load_order` now saves the system indices instead of the names as all the places using them need the system index anyway.
- The Entities `load_order` property still returns the system_names but i removed the setter as changing them directly is just asking for trouble IMHO.