core: correctly deregister QML incubators on destruction

Previously we'd try to cast the QObject* sender from
QObject::destroyed to a QQmlIncubationController*. This will always
return nullptr because C++ destructors change the type of the object
and the QQmlIncubationController destructor has already run at this
point. We now store controllers as QObject*s.

Fixes #108

src/core/generation.hpp

@@ -89,7 +89,7 @@ private slots:
 private:
 	void postReload();
 	void assignIncubationController();
-	QVector<QQmlIncubationController*> incubationControllers;
+	QVector<QObject*> incubationControllers;
 	bool incubationControllersLocked = false;
 	QHash<const void*, EngineGenerationExt*> extensions;