Revert "Set PopupView as parent of PopupWindow_QQuickView"

This reverts commit 09200b3.
That commit did fix a real problem (and a memory leak), but that problem should not occur anymore because of d18f9b3, and the commit caused other problems that are not quite trivial (musescore#22640 and musescore#22641). Therefore, we'll revert this commit for now.

src/framework/uicomponents/view/popupview.cpp

@@ -133,7 +133,7 @@ void PopupView::init()
         return;
     }
 
-    m_window = new PopupWindow_QQuickView(this);
+    m_window = new PopupWindow_QQuickView();
     m_window->init(engine, isDialog(), frameless());
     m_window->setOnHidden([this]() { onHidden(); });
     m_window->setContent(m_component, m_contentItem);