i3 config: Add mod+F3 binding to force the display off
i3 config: Add mod+F4 binding to toggle screensaver + DPMS
i3 companion: Change default notification to 1000ms

This is is milliseconds, not seconds. dunst seems to enforce a minimum
of 1 second so I didn't notice before, but I don't want to rely on that.
config/dunst: Set follow keyboard
i3 config: Spawn dmenu at the bottom of the screen