Sync behavior and documentation of QFoo::setItemDelegate()

QComboBox::setItemDelegate() does delete the old delegate which is in
constrast to all other setItemDelegate() functions which is quite
confusing. Sync in by *not* deleting the delegate in
QComboBox::setItemDelegate() and adjust the documentation to explicitly
state that the ownership is *not* passed to the affected classes.

[ChangeLog][QtWidgets][QComboBox] QComobBox::setItemDelegates no longer
deletes the previous delegate set.

Fixes: QTBUG-72483
Change-Id: I89c8e53903e7c9924a980c57b83ce40f5866e6ae
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/dialogs/qfiledialog.cpp

@@ -2015,6 +2015,9 @@ QStringList QFileDialog::history() const
     Sets the item delegate used to render items in the views in the
     file dialog to the given \a delegate.
 
+    Any existing delegate will be removed, but not deleted. QFileDialog
+    does not take ownership of \a delegate.
+
     \warning You should not share the same instance of a delegate between views.
     Doing so can cause incorrect or unintuitive editing behavior since each
     view connected to a given delegate may receive the \l{QAbstractItemDelegate::}{closeEditor()}

src/widgets/itemviews/qdatawidgetmapper.cpp

@@ -383,6 +383,9 @@ QAbstractItemModel *QDataWidgetMapper::model() const
     data from the model into the widget and from the widget to the model,
     using QAbstractItemDelegate::setEditorData() and QAbstractItemDelegate::setModelData().
 
+    Any existing delegate will be removed, but not deleted. QDataWidgetMapper
+    does not take ownership of \a delegate.
+
     The delegate also decides when to apply data and when to change the editor,
     using QAbstractItemDelegate::commitData() and QAbstractItemDelegate::closeEditor().
 

src/widgets/widgets/qcombobox.cpp

@@ -2096,6 +2096,9 @@ QAbstractItemDelegate *QComboBox::itemDelegate() const
     Sets the item \a delegate for the popup list view.
     The combobox takes ownership of the delegate.
 
+    Any existing delegate will be removed, but not deleted. QComboBox
+    does not take ownership of \a delegate.
+
     \warning You should not share the same instance of a delegate between comboboxes,
     widget mappers or views. Doing so can cause incorrect or unintuitive editing behavior
     since each view connected to a given delegate may receive the
@@ -2110,7 +2113,6 @@ void QComboBox::setItemDelegate(QAbstractItemDelegate *delegate)
         qWarning("QComboBox::setItemDelegate: cannot set a 0 delegate");
         return;
     }
-    delete view()->itemDelegate();
     view()->setItemDelegate(delegate);
 }