Add click events to i3bar

If the statusline generator (i.e. i3status) specifies click_events:true
in the protocol header, i3bar will write a JSON array on it's stdin
notifying it if the user clicks on a block.

The exact protocol is documented in docs/i3bar-protocol.

i3bar/include/child.h

@@ -33,6 +33,12 @@ typedef struct {
      * The signal requested by the client to inform it of theun hidden state of i3bar
      */
     int cont_signal;
+
+    /**
+     * Enable click events
+     */
+    bool click_events;
+    bool click_events_init;
 } i3bar_child;
 
 /*
@@ -68,4 +74,10 @@ void stop_child(void);
  */
 void cont_child(void);
 
+/*
+ * Generates a click event, if enabled.
+ *
+ */
+void send_block_clicked(int button, const char *name, const char *instance, int x, int y);
+
 #endif