ExtensionSystem: Move away from QList

Qt 6 API will move away from it.
Use QVector for API and some std container for internal things.

Change-Id: Iff14d48a47d5ac52ade875d9c8c84ad8a4f577d8
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerplugin.h

@@ -59,7 +59,7 @@ private:
     // Called from GammaRayIntegration
     Q_SLOT void getEnginesState(QByteArray *json) const;
 
-    QList<QObject *> createTestObjects() const override;
+    QVector<QObject *> createTestObjects() const override;
 };
 
 } // namespace Internal