refactor: add show manager shortcut plus refactoring

[Charlie-XIAO]

Extracted from and refactored upon #370.

This PR does the following:

• Add a show_manager shortcut for showing (and focusing) the manager window.
• Refactor shortcuts.rs, making a better macro to generate the implementations. Now each time there are only two places to modify in the backend: add an entry for Shortcuts in settings.rs, then add a key => listener pair in impl_shortcuts! in shortcuts.rs, and two places in the frontend: add the entry for Shortcuts in types/backend/settings.ts, then make the UI in manager/components/Settings/index.tsx.
• Replaced bincode with serde_json for the settings file. The reason is that, bincode seems to do well only when the structure for deserialization is not broken, otherwise bincode may directly panic (due to excessive memory allocation, etc). Why this happens? We may change the settings structure between versions. For adding fields we can put #[serde(default)] to avoid errors. For removing field, serde_json automatically ignores redundant stuff (it seems so). For changing fields, we can make some kind of deprecation cycle. bincode does not seem to be flexible enough when dealing with these. Also our settings won't be so huge that serialization/deserialization performance differ a lot, so settings.json would be just fine.