Add Mod4 (Windows) key

Been sitting on this for quite a while, thought I'd try to get it upstream so I don't need my own build anymore.

I'm using squeekboard in sway so my overall workflow is usually completely keyboard driven. I also use a convertible and want to be able to use it completely without the hardware keyboard if necessary. For that, I need access to the Mod4 key so I can use my keyboard shortcuts.

Thanks a lot for providing the only wayland on screen keyboard that isn't tied to some specific desktop environment (at least the only one I'm aware of)!