Add window focused signal

[anriha]

This PR adds a new signal that gets triggered whenever a window receives keyboard focus. I've tested the core functionality and it works as expected, though I haven't tested the Lua API yet (it's a bit tricky to set up on NixOS).

This is just the first of several signals I'm planning to add. My goal is to implement enough signals to replicate the dwl format for yambar integration, as I'd like to make pinnacle my daily driver.

Personal note: I'm really excited about this project! I've tried many Wayland compositors, and was actually planning to write my own because each existing one was missing something crucial. Then I found pinnacle, and it's exactly what I was looking for. Really happy to be able to contribute.

[Ottatop]

Oh hey I was thinking about this a few days ago

Implementation looks good! Did a sanity check and it does work on the Lua side. Just a minor comment nit.

I was considering whether or not we could extend this to something like fn(Option<&WindowHandle>, Option<&WindowHandle>) and give people both the old and new focuses as well as notifying on focus loss, but I'm not sure how to handle the case where a window gets closed. In that case I don't really want to pass a dead window handle to the signal because it's not useful. We can roll with this until something gets figured out.

[anriha]

Yeah, I thought about that possibility too. Definitely more edge cases to consider there, especially with the closed window scenario you mentioned. And honestly, I suspect the use case for having both old/new focus info would be pretty rare in practice. But I'll keep it in mind while I'm working on other signals - might stumble upon a clean solution along the way.

[Ottatop]

Aight lgtm, thank you!