Expose “/route/trim” midi binding.

libs/ardour/session_state.cc

@@ -3077,6 +3077,10 @@ Session::controllable_by_descriptor (const ControllableDescriptor& desc)
 		c = r->gain_control ();
 		break;
 
+	case ControllableDescriptor::Trim:
+		c = r->trim()->gain_control ();
+		break;
+
 	case ControllableDescriptor::Solo:
                 c = r->solo_control();
 		break;

libs/pbd/controllable_descriptor.cc

@@ -72,6 +72,9 @@ ControllableDescriptor::set (const std::string& str)
 	if (path[1] == "gain") {
 		_subtype = Gain;
 
+	} else if (path[1] == "trim") {
+		_subtype = Trim;
+
 	} else if (path[1] == "solo") {
 		_subtype = Solo;
 

libs/pbd/pbd/controllable_descriptor.h

@@ -36,6 +36,7 @@ public:
 
     enum SubType {
 	    Gain,
+	    Trim,
 	    Solo,
 	    Mute,
 	    Recenable,