Implement app grid navigation as designed in https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/appgrid/appgrid-dnd.png.
WIP at the moment as it: Lacks the arrows in the mockup, assets welcome
- Some parts of the code are crazy, esp. those related to the transparent areas on the sides
Kinda related to the last point I see artifacts on the left side one, couldn't track yet what they are about.
Fixes: #3574 (closed)