add signal for automation state changes per plugin

libs/ardour/ardour/automatable.h

@@ -81,6 +81,8 @@ public:
 	int set_automation_xml_state (const XMLNode&, Evoral::Parameter default_param);
 	XMLNode& get_automation_xml_state();
 
+	PBD::Signal0<void> AutomationStateChanged;
+
   protected:
 	Session& _a_session;
 

libs/ardour/automatable.cc

@@ -278,6 +278,7 @@ Automatable::set_parameter_automation_state (Evoral::Parameter param, AutoState
 	if (c && (s != c->automation_state())) {
 		c->set_automation_state (s);
 		_a_session.set_dirty ();
+		AutomationStateChanged(); /* Emit signal */
 	}
 }