Reset parentItem when mainItem changes

Otherwise we'll keep piling up custom tooltip items in the dialog. They're invisible but they're still there.

Differential Revision: https://phabricator.kde.org/D17542

src/plasmaquick/dialog.cpp

@@ -783,6 +783,7 @@ void Dialog::setMainItem(QQuickItem *mainItem)
         if (d->mainItem) {
             disconnect(d->mainItem, nullptr, this, nullptr);
             d->mainItem->setVisible(false);
+            d->mainItem->setParentItem(nullptr);
         }
 
         if (d->mainItemLayout) {