util: Signal

Signal is an enhanced version of EventEmitter:

* only the owner can emit a signal (aka trigger an event)
* signal connection (aka event subscription) can be disconnected

EventEmitter is deprecated in favor of Signal.

refs #2279

Change-Id: I74ea5fef2e1e9b34776aa04f01170600b171152e
diff --git a/src/util/event-emitter.hpp b/src/util/event-emitter.hpp
index 261dabc..bcb703b 100644
--- a/src/util/event-emitter.hpp
+++ b/src/util/event-emitter.hpp
@@ -39,6 +39,8 @@
  *    onEventName(args);
  *  To clear event subscriptions:
  *    onEventName.clear();
+ *
+ *  \deprecated use Signal instead
  */
 
 template<typename ...TArgs>