Add an OSC message that creates a marker with a given name.

--- 6.0-rc1/libs/surfaces/osc/osc.cc	2020-05-12 16:28:40.000000000 +0100
+++ 6.0-rc1/libs/surfaces/osc/osc.cc	2020-05-12 16:32:59.539041629 +0100
@@ -435,6 +435,7 @@
 		REGISTER_CALLBACK (serv, X_("/surface/list"), "f", surface_list);
 		REGISTER_CALLBACK (serv, X_("/add_marker"), "", add_marker);
 		REGISTER_CALLBACK (serv, X_("/add_marker"), "f", add_marker);
+		REGISTER_CALLBACK (serv, X_("/add_marker"), "s", add_marker_name);
 		REGISTER_CALLBACK (serv, X_("/access_action"), "s", access_action);
 		REGISTER_CALLBACK (serv, X_("/loop_toggle"), "", loop_toggle);
 		REGISTER_CALLBACK (serv, X_("/loop_toggle"), "f", loop_toggle);
--- 6.0-rc1/libs/surfaces/osc/osc.h	2020-05-12 16:28:40.000000000 +0100
+++ 6.0-rc1/libs/surfaces/osc/osc.h	2020-05-12 16:31:26.400215284 +0100
@@ -343,6 +343,10 @@
 	void transport_speed (lo_message msg);
 	void record_enabled (lo_message msg);
 
+	void add_marker_name(const std::string &markername) {
+		add_marker(markername);
+	}
+
 	// cue
 	Sorted cue_get_sorted_stripables(boost::shared_ptr<ARDOUR::Stripable> aux, uint32_t id, lo_message msg);
 	int cue_parse (const char *path, const char* types, lo_arg **argv, int argc, lo_message msg);
@@ -483,6 +487,7 @@
 	}
 
 	PATH_CALLBACK1(set_transport_speed,f,);
+	PATH_CALLBACK1(add_marker_name,s,&);
 	PATH_CALLBACK1(access_action,s,&);
 
 	PATH_CALLBACK1(jump_by_bars,f,);