Add a way to update the wallpaper of a running instance

[Kasper24]

I've made my own GUI wrapping this great tool in AwesomeWM, but the issue I found is that when wallpaper engine draws to the root window, Awesome lags quite a bit, and its CPU usage also goes up to around 5-10% depending on the wallpaper and FPS. I'm not sure if it's only an issue in Awesome that doesn't like things consistently drawing to the root window, or is it an X11 thing.
Anyways, my solution for this is somewhat of a hack - I run wallpaper engine in a regular window, then resize it to the screen size, make it float, make it invisible in the taskbar, etc - you get the point. The annoying thing is that I have to consistently make a new instance for it and kill the old one.
I thought it could be cool that rather than having to kill the old instance and spawn a new window each time, it would be possible to update the existing window. Possibly via some DBus (or other IPC) interface? It could also be used to change everything that's now customizable via the command line args, maybe ask for available wallpapers, and get their JSON data. This would make it really easy to integrate this tool into other tools, make plugins for other DEs/WMs, etc.

Unrelated, but another thing that would be nice is being able to pass in the X11 class as an argument since right now you have it set dynamically based on the wallpaper title, which makes it hard to detect it based on the class. I saw you already have the X11 name set to the wallpaper title, so I think by default it would be saner to have the class be something like 'linux-wallpaperengine' or whatever you prefer. The same goes for x, y, width, and height I've made a PR for that.