Add BackSpace bindings to kill window or session

dotfiles/i3/config

 bindsym XF86AudioLowerVolume exec "saturnin au down"
 bindsym XF86AudioMute        exec "saturnin au flip"
 
+bindsym $mod+BackSpace       kill
+bindsym $mod+Shift+BackSpace exec "i3-nagbar -t warning -m 'You pressed the exit shortcut. Do you really want to exit i3? This will end your X session.' -b 'Yes, exit' 'i3-msg exit'"
+
 mouse_warping none