Make action label string with & for explicit accelerator.

Due to explicitly using Kirigami.MnemonicData in ActionListItem, the
i18n string should explicitly set the accelerator. Otherwise it won't
work propertly for locale like CJK.

For example, &Settings should be translated to 设置(&S) to ensure
accelerator is preserved. Without explicit accelerator translator won't
know it can be accessed with accelerator.