Implement IPC binding event (keyboard)

This implements the IPC binding event for keyboard bindings.  It is
slightly different from the i3 implementation [1] since sway supports
more than one non-modifier key in a binding. Thus the json interface has
been changed from:

    {
      ...
      "symbol": "t",
      ...
    }

to:

    {
      ...
      "symbols": [ "t" ],
      ...
    }

[1] http://i3wm.org/docs/ipc.html#_binding_event

include/ipc-server.h

@@ -21,6 +21,10 @@ void ipc_event_mode(const char *mode);
  * the name of that modifier.
  */
 void ipc_event_modifier(uint32_t modifier, const char *state);
+/**
+ * Send IPC keyboard binding event.
+ */
+void ipc_event_binding_keyboard(struct sway_binding *sb);
 const char *swayc_type_string(enum swayc_types type);
 
 #endif