Improve how systems are defined and organized

[richardbiely]

Current version:
System is a virtual class with overridden member functions. Systems are managed by SystemManager. They are not connected. On the outside this looks very similar to what DOTS does.

class PositionSystem final: public ecs::System {
  static constexpr float dt = 0.01f;
	ecs::Query m_q;

public:
	void OnCreated() override {
		m_q = world().query().all<Position&, Velocity>();
	}

	void OnUpdate() override {
		m_q.each([](Position& p, const Velocity& v) {
			p.x += v.x * dt;
			p.y += v.y * dt;
			p.z += v.z * dt;
		});
	}
};

ecs::SystemManager sm(w);
sm.add<PositionSystem>();
sm.add<OtherSystem>();
...
sm.update(); // run all systems once

Proposed version:
Systems are entities with a System component attached. Maybe even inheriting from Query (so they are cached). This gives us a lot of flexibility. Enabling the system = enabling the entity. Systems can be queried (e.g. disable all systems that do this-and-that easily).
Organized into stages. Stages can be user defined, probably implemented as components. In order for the grouping to work correctly, relationship support would come handy.

auto s = w.system()
        .stage<LateUpdate>() // optional, if not used, the system goes to the default stage
	.all<Position&, Velocity>()
	.any<Rotation>()
	.OnCreated([](ecs::Query& q) { // event when created
	})
	.OnStopped([](ecs::Query& q) { // event when stopped
		...
	})
       // other events
        ...
       // function to run every update (equivalent to ecs::query::each)
	.OnUpdate([](Position& p, const Velocity& v) {
		p.x += v.x * dt;
		p.y += v.y * dt;
		p.z += v.z * dt;
	})
	.commit();

Necessary:

• support for entity naming
• entity sorting/grouping
• entity relationship support