improvements to EntityManagers and Filament APIs (#7302)

* prevent public classes from being created on the stack

- we used to to this by deleting operator delete, but this prevented
  the internal "F" classes from being virtual; which can be useful
  when using EntityManger::Listener.
  now we just make the destructor protected in each class.

- EntityManger::Listener now has a virtual destructor so that
  objects could be correctly destroyed from Listener*

* improve EntityManger and Component managers

- all component managers now have the same "base" API
    - getComponentCount()
	- empty()
    - getEntity()
    - getEntities()

- Scene now has getEntityCount()

- EntityManager now has getEntityCount()

- all component manager implement gc() the same way, by calling destroy()

- SingleInstanceComponentManager::gc() that calls removeComponent() has
  been removed because it's dangerous. removeComponent() is often
  not enough, some additional cleanup might be needed.

libs/utils/src/EntityManager.cpp

@@ -22,6 +22,8 @@
 
 namespace utils {
 
+EntityManager::Listener::~Listener() noexcept = default;
+
 EntityManager::EntityManager()
         : mGens(new uint8_t[RAW_INDEX_COUNT]) {
     // initialize all the generations to 0
@@ -32,8 +34,6 @@ EntityManager::~EntityManager() {
     delete [] mGens;
 }
 
-EntityManager::Listener::~Listener() noexcept = default;
-
 EntityManager& EntityManager::get() noexcept {
     // note: we leak the EntityManager because it's more important that it survives everything else
     // the leak is really not a problem because the process is terminating anyway.
@@ -57,6 +57,10 @@ void EntityManager::unregisterListener(EntityManager::Listener* l) noexcept {
     static_cast<EntityManagerImpl *>(this)->unregisterListener(l);
 }
 
+size_t EntityManager::getEntityCount() const noexcept {
+    return static_cast<EntityManagerImpl const *>(this)->getEntityCount();
+}
+
 #if FILAMENT_UTILS_TRACK_ENTITIES
 std::vector<Entity> EntityManager::getActiveEntities() const {
     return static_cast<EntityManagerImpl const *>(this)->getActiveEntities();