Always sort monitors according to their X offsets
Add placement{save,restore}

Prior to changing my monitor layout using xrandr, I want to have the
option to save the current layout: Current monitor and tags of each
client, plus the selected tags on each monitor. At some later point in
time, most likely after restoring an earlier screen configuration, I
want to restore that saved layout.

This allows me to temporarily unplug my laptop from stationary screens,
run through the office, come back, and then restore my previous work
layout.