i3bar: properly handle the _XEMBED_INFO property

i3bar/include/trayclients.h

@@ -17,6 +17,7 @@ TAILQ_HEAD(tc_head, trayclient);
 
 struct trayclient {
     xcb_window_t       win;         /* The window ID of the tray client */
+    bool               mapped;      /* Whether this window is mapped */
 
     TAILQ_ENTRY(trayclient) tailq;  /* Pointer for the TAILQ-Macro */
 };