fix: Un-register component keys down the component tree (#2792)

ComponentKey for children components were not getting unregistered when
the sub-tree was removed from the game. As a result:

- same key couldn't be reused
- components with such keys couldn't be readded once removed
- component with such keys were not being garbage collected (didn't
actually verify this one though)

packages/flame/lib/src/components/core/component.dart

@@ -896,17 +896,12 @@ class Component {
   void _remove() {
     assert(_parent != null, 'Trying to remove a component with no parent');
 
-    if (_key != null) {
-      final game = findGame();
-      if (game is FlameGame) {
-        game.unregisterKey(_key!);
-      }
-    }
     _parent!.children.remove(this);
     propagateToChildren(
       (Component component) {
         component
           ..onRemove()
+          .._unregisterKey()
           .._clearMountedBit()
           .._clearRemovingBit()
           .._setRemovedBit()
@@ -920,6 +915,15 @@ class Component {
     );
   }
 
+  void _unregisterKey() {
+    if (_key != null) {
+      final game = findGame();
+      if (game is FlameGame) {
+        game.unregisterKey(_key!);
+      }
+    }
+  }
+
   //#endregion
 
   //#region Debugging assistance

packages/flame/test/components/component_test.dart

@@ -1381,6 +1381,29 @@ void main() {
         },
       );
 
+      testWithFlameGame(
+        'Removed keys can be reused by components',
+        (game) async {
+          final key = ComponentKey.named('A');
+          final parent1 = Component(children: [ComponentA(key: key)]);
+
+          game.world.add(parent1);
+          await game.ready();
+
+          parent1.removeFromParent();
+          await game.ready();
+
+          final component = ComponentA(key: key);
+          final parent2 = Component(children: [component]);
+
+          game.world.add(parent2);
+          await game.ready();
+
+          final retrieved1 = game.findByKey(key);
+          expect(retrieved1, equals(component));
+        },
+      );
+
       testWithFlameGame(
         'Throws assertion error when registering a component with the same key',
         (game) async {