[feat] [macOS] App focus gain/loss handlers

Describe the problem

I am building a shortcut system for my desktop app, for triggering actions using hotkeys that aren't in any menu. From what I understand, there are two ways to go about this currently:

1. Register keydown js listeners in the webview
2. Register global key listeners on the Rust side using something like rdev.

Both solutions expose a problem on macOS: An application can have focus without having any windows open. For example: without any windows open, I would still like to be able to listen for Command + N, to open a new window.

1. For JS, I'm registering key listeners directly on the window, but there's no way of listening to non-window key pressed afaik.
2. For Rust, I can register global OS shortcuts (using rdev), but I would need to unregister these if the app loses focus. On macOS, this is not the same as checking if any window has focus.

In lieu of an actual key event handler solution on the Rust side (one that uses the app event loop itself), I'm inclined to go for solution 2, but I would need to know if my app has focus or not.

Describe the solution you'd like

Ideally, we could install an app focus loss/gain handler on the App builder, something along these lines:

tauri::Builder::default()
    .on_app_focus_gain(|app|{ }))
    .on_app_focus_loss(|app|{ }))
    .run()

These lambda's would directly map to NSApplicationDelegate::applicationDidBecomeActive() and its counterpart.

Alternatives considered

In this particular case I can put Command + N in my main application menu, and it will get triggered, but that is a happy coincidence. This isn't always the case.

Additional context

• I would like to point out these handlers are useful outside of the scope of a shortcut system. Maybe someone wants to shutdown a thread while the app is inactive, or something like that.
• Ideally I wouldn't even have to use rdev to listen for hotkeys and use a Tauri-native solution, but that's a different discussion.

Start at the Tauri App builder and its macOS application delegate integration, then compare the requested callbacks with NSApplicationDelegate::applicationDidBecomeActive() and its counterpart. Done means app focus gain and loss handlers can be registered on the builder and are invoked when the application becomes active or inactive.