startup-notifications: keep sequence around for 30s after completion

This changes the fact that Firefox would not be launched on the correct
workspace because it marked the startup sequence as completed *before*
actually mapping all of its windows.

To test this, go to workspace 3 and run this command in a terminal:
    i3-msg 'exec iceweasel; workspace 4'
That will make i3 start iceweasel (and create a proper startup
notification context for it), then immediately switch to workspace 4
(before iceweasel could possibly start).

The iceweasel window(s) should appear on workspace 3.

include/data.h

@@ -168,6 +168,9 @@ struct Startup_Sequence {
     char *workspace;
     /** libstartup-notification context for this launch */
     SnLauncherContext *context;
+    /** time at which this sequence should be deleted (after it was marked as
+     * completed) */
+    time_t delete_at;
 
     TAILQ_ENTRY(Startup_Sequence) sequences;
 };