add generic ::slaved() method to Route

libs/ardour/ardour/route.h

@@ -564,6 +564,7 @@ public:
 			pframes_t nframes, int declick);
 
         bool slaved_to (boost::shared_ptr<VCA>) const;
+        bool slaved () const;
 
   protected:
         friend class Session;

libs/ardour/route.cc

@@ -5198,6 +5198,16 @@ Route::master_send_enable_controllable () const
 #endif
 }
 
+bool
+Route::slaved () const
+{
+	if (!_gain_control) {
+		return false;
+	}
+	/* just test one particular control, not all of them */
+	return _gain_control->slaved ();
+}
+
 bool
 Route::slaved_to (boost::shared_ptr<VCA> vca) const
 {