feat(api): get_state_label returns the state label for UI display

[lotem]

enable the frontend to display a message when an arbitrary option defined in the
schema is updated.

Closes #447

the option name eg. ascii_mode is passed to the notification handler function,
there, get_state_label gives IME frontend the access to the human readable label
for the current state of the option defined in .schema:/switches/@/states.

when selection is changed in a radio group, more than one option in the group is
updated so it only returns a non-empty label while querying the selected option.

Pull request

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1. Unit and manual test pass
2. GitHub Action CI pass
3. At least one contributor reviews and votes
4. Can be merged clean without conflicts
5. PR will be merged by rebase upstream base

Additional Info

[lotem]

Ignore the indentation change.

Only added a new function to the API at the end of the RimeApi init block.

[LEOYoon-Tsaw]

It seems it is non trivial to implement this in squirrel, the session is handled in SquirrelInputController, which is a private class, while notification handling is done in SquirrelApplicationDelegate

[lotem]

It seems it is non trivial to implement this in squirrel, the session is handled in SquirrelInputController, which is a private class, while notification handling is done in SquirrelApplicationDelegate

Session ID is given as an argument of the notification handler callback.
See the example code in tools/rime_api_console.cc

[lotem]

应该还有后续啊。

状态选项里的单选状态组现在不支持通过快捷键切换，用不上这么好的文字提示，我准备把快捷键单选功能做好。

另外，我打算把 Control+Shift+2 等快捷键
https://github.com/rime/rime-prelude/blob/master/key_bindings.yaml#L45

    - { when: always, accept: Control+Shift+2, toggle: ascii_mode }
    - { when: always, accept: Control+Shift+3, toggle: full_shape }
    - { when: always, accept: Control+Shift+4, toggle: simplification }
    - { when: always, accept: Control+Shift+5, toggle: extended_charset }

重新定义为

    - { when: always, accept: Control+Shift+2, toggle: '@0' }
    - { when: always, accept: Control+Shift+3, toggle: '@1' }
    - { when: always, accept: Control+Shift+4, toggle: '@2' }
    - { when: always, accept: Control+Shift+5, toggle: '@3' }

指改变方案中 switches/@0 等处定义的状态。这样的配置就通用了。兼容方案自定义的状态。

@x 如果不是开关而是单项选择状态组，最好不必改动词 toggle，让他轮换到下一个状态。免得还得专门改快捷键配置。

[LEOYoon-Tsaw]

如果是狀態組，可以用set_option、unset_option來設定快捷鍵吧，當然加個輪換功能也是很好的