i3bar: Add default bar_id

Instead of erroring, request the list of bar configs from i3 and use the
first one.

i3bar/include/configuration.h

@@ -74,6 +74,13 @@ extern config_t config;
  */
 void parse_config_json(char *json);
 
+/**
+ * Start parsing the received bar configuration list. The only usecase right
+ * now is to automatically get the first bar id.
+ *
+ */
+void parse_get_first_i3bar_config(char *json);
+
 /**
  * free()s the color strings as soon as they are not needed anymore.
  *