Add method to check if Session is empty

Empty means that it contains some tracks or user-addedd
busses or VCA (not counting master, monitor, surround busses).

This is motivated by A&H control-surface.

libs/ardour/ardour/session.h

@@ -359,6 +359,7 @@ public:
 	uint32_t ntracks () const;
 	uint32_t naudiotracks () const;
 	uint32_t nbusses () const;
+	bool     empty () const;
 
 	bool plot_process_graph (std::string const& file_name) const;
 

libs/ardour/luabindings.cc

@@ -3146,6 +3146,7 @@ LuaBindings::common (lua_State* L)
 		.addFunction ("name", &Session::name)
 		.addFunction ("path", &Session::path)
 		.addFunction ("uuid", &Session::uuid)
+		.addFunction ("empty", &Session::empty)
 		.addFunction ("record_status", &Session::record_status)
 		.addFunction ("maybe_enable_record", &Session::maybe_enable_record)
 		.addFunction ("disable_record", &Session::disable_record)

libs/ardour/session.cc

@@ -6736,6 +6736,20 @@ Session::nstripables (bool with_monitor) const
 	return rv;
 }
 
+bool
+Session::empty() const
+{
+	StripableList sl;
+	get_stripables (sl);
+	for (auto const& s: sl) {
+		if (s->is_singleton ()) {
+			continue;
+		}
+		return false;
+	}
+	return true;
+}
+
 bool
 Session::plot_process_graph (std::string const& file_name) const {
 	return _graph_chain ? _graph_chain->plot (file_name) : false;