~/.config/kanshi: Manage (external) monitors with kanshi

This supports dynamic plug/unplug of monitors, e.g. switching to the
external monitor if it is present, and to the internal display if no
external monitor is connected.

I could not figure out yet how to start up kanshi automatically together
with sway, so right now I only start it when I need it.  (Sway's default
is to use all monitors, which works well.)

.config/kanshi/config

profile external {
	output DP-1 enable mode 2560x1440 position 0,0
	output LVDS-1 disable
}
profile laptop {
	output LVDS-1 enable
}