Fix propagation of QContextMenuEvent for QQuickItems

QQuickDeliveryAgentPrivate::deliverContextMenuEvent() relies on that the
QContextMenuEvent gets ignored if the target item doesn't handle it,
therefore we have to make sure it is ignored by default.

This fixes the context menu for the SideBar (still WIP) and likewise the
autotest in this change.

Pick-to: 6.9
Change-Id: If72a814adc92aea6e5e071eb49cbe84b025768fb
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/quick/items/qquickitem.cpp

@@ -9184,6 +9184,8 @@ bool QQuickItem::event(QEvent *ev)
         break;
     case QEvent::ContextMenu:
         // ### Qt 7: add virtual contextMenuEvent (and to QWindow?)
+        ev->ignore();
+        break;
     default:
         return QObject::event(ev);
     }

tests/auto/quickcontrols/qquickcontextmenu/data/customContextMenuOnNestedRectangle.qml

@@ -0,0 +1,39 @@
+import QtQuick
+import QtQuick.Controls
+
+ApplicationWindow {
+    width: 600
+    height: 400
+    //visible: true // for manual testing
+
+    Rectangle {
+        objectName: "tomato"
+        x: 100
+        y: 100
+        width: 100
+        height: 100
+        color: "tomato"
+        radius: width / 2
+
+        ContextMenu.menu: Menu {
+            id: contextMenu
+            MenuItem {
+                text: qsTr("Eat tomato")
+            }
+            MenuItem {
+                text: qsTr("Throw tomato")
+            }
+            MenuItem {
+                text: qsTr("Squash tomato")
+            }
+        }
+        Rectangle {
+            objectName: "orange"
+            anchors.centerIn: parent
+            width: 80
+            height: 80
+            color: "orange"
+            radius: width / 2
+        }
+    }
+}

tests/auto/quickcontrols/qquickcontextmenu/tst_qquickcontextmenu.cpp

@@ -47,6 +47,7 @@ void tst_QQuickContextMenu::customContextMenu_data()
 
     QTest::addRow("Rectangle") << "customContextMenuOnRectangle.qml";
     QTest::addRow("Control") << "customContextMenuOnControl.qml";
+    QTest::addRow("NestedRectangle") << "customContextMenuOnNestedRectangle.qml";
 }
 
 void tst_QQuickContextMenu::customContextMenu()