Add support for a relative mouse.

[A2Wolverin3]

Adds support for a relative mouse.

I needed this for my NVR which doesn't recognize absolute position pointer. This adds new menu item in the 'View' menu which lets users enter a pointerlock-ed "relative" mouse mode. This mode must be exited with the Escape key.

Fixes #853

[mtlynch]

This is great! Thanks for contributing this. You did a great job of matching our conventions around style and testing.

This is a pretty big feature, and it's going to require some iterations on architecting the different pieces of it and adjusting the web UI. Honestly, it is going to be a significant amount of work, so I don't want you to feel obligated to go through that process with us, but I also don't want to cut you out of the process if you want to do that heavy lifting.

Would you prefer we take the PR from here and iterate on it internally? Or would you prefer that we give you feedback what architecture and UI changes we'd need to see?

[A2Wolverin3]

You can go ahead and iterate without me. To be honest, I'm mostly just trying to get this working in my smarthome setup where I've got my NVR tucked away in a not-very-accessible place. But my cheap NVR is particular and needed a relative mouse. (And annoyingly, the absence of an absolute mouse.) I knew this was an item on the list, so I figured I'd give it a go and send it in.

I'm also docker-izing things for my setup. (I can send a PR for that as well if you like, but I gather from reading past issues and questions that docker isn't something that's quite on your roadmap for this project.) And since I'm pretty sure the general audience isn't looking to run with my custom smarthome configuration, I'll probably be pulling and running from my own fork anyway. 😉

[mtlynch]

Okay, cool.

I'm going to close this PR for now, but I've prioritized the feature for our next sprint.

[mtlynch]

Oh, forgot to respond to this part:

I'm also docker-izing things for my setup. (I can send a PR for that as well if you like, but I gather from reading past issues and questions that docker isn't something that's quite on your roadmap for this project.) And since I'm pretty sure the general audience isn't looking to run with my custom smarthome configuration, I'll probably be pulling and running from my own fork anyway.

Yeah, there was some prior work around Docker in #403 but we're not planning to adopt Docker for the foreseeable future. Our worry is that by adding another layer of abstraction, it's an extra step in debugging to figure out if it's an inherent bug or a consequence of our Docker packaging.

I really like Docker, but for this project, we've already spent our complexity budget elsewhere.