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/common.h

@@ -54,6 +54,10 @@ struct status_block {
     uint32_t x_offset;
     uint32_t x_append;
 
+    /* Optional */
+    char *name;
+    char *instance;
+
     TAILQ_ENTRY(status_block) blocks;
 };