[kcm/kwindesktop] Emit rowsChanged signal to fix default state highlighting

Without this, the rowsChanged signal wasn't emitting at startup, so the
binding `highlight: kcm.desktopsModel.rows !== 2` didn't work correctly.

FIXED-IN: 5.24.7 5.25.3 5.26

src/kcmkwin/kwindesktop/desktopsmodel.cpp

@@ -457,6 +457,8 @@ void DesktopsModel::getAllAndConnect(const QDBusMessage &msg)
         m_names = m_serverSideNames;
         m_rows = m_serverSideRows;
         endResetModel();
+
+        Q_EMIT rowsChanged();
     }
 
     Q_EMIT readyChanged();