Add custom buttons to navigation bar

[Jake-Jensen]

I think the ability to have custom buttons or shortcuts would be neat and useful, such as left/right arrows, custom app shortcuts, or custom events, like Samsung's Goodlock / NavStar.

Is this feature implemented on any other app/mod/rom? please specify
Goodlock, multiple old cyanogen ROMs, MIUI mod ROMs, and there were a few Magisk mods that did this.