QmlDesigner: Fix assets library drag glitch

When clicking an asset and then moving the mouse, drag is still
active. Happens on mac and linux only.

Change-Id: Ic61a9316013271e8e16a30629416787c863840a3
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>

src/plugins/qmldesigner/components/assetslibrary/assetslibrarywidget.cpp

@@ -91,6 +91,8 @@ bool AssetsLibraryWidget::eventFilter(QObject *obj, QEvent *event)
                 m_assetsToDrag.clear();
             }
         }
+    } else if (event->type() == QMouseEvent::MouseButtonRelease) {
+        m_assetsToDrag.clear();
     }
 
     return QObject::eventFilter(obj, event);