Add a timeout: delay_exit_on_zero_displays

Outputs may disappear momentarily and come back later.
To prevent i3 from exit when no output is available momentarily, add a timeout delay_exit_on_zero_displays.

include/config.h

@@ -167,6 +167,10 @@ struct Config {
      * flag can be delayed using an urgency timer. */
     float workspace_urgency_timer;
 
+    /** Use a timer to delay exiting when no output is available.
+      * This can prevent i3 from exiting when all outputs disappear momentarily. */
+    float zero_disp_exit_timer_ms;
+
     /** Behavior when a window sends a NET_ACTIVE_WINDOW message. */
     enum {
         /* Focus if the target workspace is visible, set urgency hint otherwise. */