Add "mode" field in binding event

This does *not* go in the binding object to reflect the same hierarchy
of the config file: a mode is a collection of bindings.

Fixes #5323

include/ipc.h

@@ -133,7 +133,7 @@ void ipc_send_barconfig_update_event(Barconfig *barconfig);
 /**
  * For the binding events, we send the serialized binding struct.
  */
-void ipc_send_binding_event(const char *event_type, Binding *bind);
+void ipc_send_binding_event(const char *event_type, Binding *bind, const char *modename);
 
 /**
  * Set the maximum duration that we allow for a connection with an unwriteable