util: add isConnected() in signal::Connection and signal::ScopedConnection

Change-Id: Iddf2a084ccf137a691cd85c54003182acf0e5afd
Refs: #2308
diff --git a/src/util/signal-connection.hpp b/src/util/signal-connection.hpp
index 1276182..f2ac45e 100644
--- a/src/util/signal-connection.hpp
+++ b/src/util/signal-connection.hpp
@@ -45,6 +45,12 @@
   void
   disconnect();
 
+  /** \brief check if connected to the signal
+   *  \return false if disconnected from the signal
+   */
+  bool
+  isConnected() const;
+
   /** \brief compare for equality
    *
    *  Two connections are equal if they both refer to the same connection that isn't disconnected,