add: TimerSystem. Timer-based event handling API.

[sagrig]

Implements #64

[sagrig]

The current callback signature 'bool(void)' is a draft. Further commit should generalize it, considering:

• callbacks may be invoked explicitly (i.e. outside TimerSystem::runEvents())
• callbacks may accept arguments and return objects of various types

[cfnptr]

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[cfnptr]

In Entity Component System (ECS) paradigm this should be a TimerComponent, please see the ComponentSystem<> usage. With current implementation we can't attach this timer to an entity.

[cfnptr]

This should be private, because it's subscribed to the "Timer" event which managed by the Manager.

[cfnptr]

All systems have built-in virtual disposeComponents() function, please see the ComponentsSystem<> for reference.

[cfnptr]

Please see the ComponentsSystem<> disposeComponents() function for reference.

[cfnptr]

The current callback signature 'bool(void)' is a draft. Further commit should generalize it, considering:

* callbacks may be invoked explicitly (i.e. outside TimerSystem::runEvents())

* callbacks may accept arguments and return objects of various types

I think we should use strings for timer function callback, like it's done in the PhysicsSystem for sensor collider events:

garden/include/garden/system/physics.hpp

Line 364 in eeab52b

string eventListener; /**< Rigidbody events listener name. */

garden/source/system/physics.cpp

Lines 1236 to 1242 in eeab52b

	if (entity1)
	{
	auto rigidbodyView = getComponent(entity1);
	toEventName(eventName, rigidbodyView->eventListener, bodyEvent.eventType);
	thisBody = entity1; otherBody = entity2;
	manager->tryRunEvent(eventName);
	}