[System Tray] Sort plasmoids in "Extra Items" by name

BUG: 386445

Differential Revision: https://phabricator.kde.org/D8615

applets/systemtray/systemtray.cpp

@@ -450,6 +450,7 @@ QAbstractItemModel* SystemTray::availablePlasmoids()
             item->setData(info.pluginName());
             m_availablePlasmoidsModel->appendRow(item);
         }
+        m_availablePlasmoidsModel->sort(0 /*column*/);
     }
     return m_availablePlasmoidsModel;
 }