feat: add option shown_notifications_when: {string}, to determine when to show notifications

[fxliang]

by this pr, we can determine when to show notifications, except deployment's.

for example, set show_notifications_when: never could disable all notifications except msg from deployment.

#in weasel.yaml

# configuration: when to show notifications
# always, never, or combination of: ascii_mode, ascii_punct, shape, simplification, schema
# e.g., ascii_mode+shape+schema
show_notifications_when: always

[oniondelta]

能否像 Mac 鼠鬚管一樣，自設的 switches 項目也可顯示提示？
如果能就太好了🙏🏻！
一直想提這樣的設定，不然 Win 上快捷鍵切換，好似盲切！

[fxliang]

能否像 Mac 鼠鬚管一樣，自設的 switches 項目也可顯示提示？ 如果能就太好了🙏🏻！ 一直想提這樣的設定，不然 Win 上快捷鍵切換，好似盲切！

#1055 已实现

[oniondelta]

非常感謝🙏🏻 👍🏻