Add spacing between tiled windows
See https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/mobile-shell/mobile-shell-convergence.png . Filing this separate from #165 since the current left/right tiling would already benefit.
See https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/mobile-shell/mobile-shell-convergence.png . Filing this separate from #165 since the current left/right tiling would already benefit.