Set parentItem for pages in DockWindow

After https://invent.kde.org/qt/qt/qtdeclarative/-/merge_requests/40 a
QQuickItem without a parentItem is always invisible. While this avoids
some crashes in plasmashell, it breaks some applications like MuseScore
where pages don't have a parent item.

This is also important for Qt6 porting.

Fixes musescore#17276

src/appshell/view/dockwindow/dockwindow.cpp

@@ -619,6 +619,7 @@ void DockWindow::initDocks(DockPageView* page)
     }
 
     if (page) {
+        page->setParentItem(this);
         page->init();
     }