Detect X Server shutdown or document how to end succade

[domsson]

Currently, without taking proper precautions, succade might keep on running even if the X Server is being shut down. We should research if there is a way to detect when X (or alternatively succade's parent process) is gone. If that's not reliably possible, we should document this somehow (for example, add an example that shows how to modify the sxhkd hotkeys to also shut down succade properly.

[domsson]

We might be able to use getenv("DISPLAY"), as $DISPLAY should not be set when X isn't running. Haven't looked into it in detail yet, though.