Handle TouchCancel events in MultiPointTouchArea.

Ensure canceled has been emitted and the touch point press states have
been reset after a touch cancel event.

Change-Id: I421e8e4bdc6793b5ddc4b905ffecef91cf61ed9d
Reviewed-by: Martin Jones <martin.jones@qinetic.com.au>

src/quick/items/qquickmultipointtoucharea.cpp

@@ -412,6 +412,12 @@ void QQuickMultiPointTouchArea::touchEvent(QTouchEvent *event)
         }
         break;
     }
+    case QEvent::TouchCancel:
+        _stealMouse = false;
+        setKeepMouseGrab(false);
+        setKeepTouchGrab(false);
+        ungrabTouchPoints();
+        break;
     default:
         QQuickItem::touchEvent(event);
         break;