add a user config var for preferred time domain (libardour part)

(as opposed to default_time_domain which is the per-session default)

libs/ardour/ardour/rc_configuration_vars.h

@@ -30,6 +30,8 @@
     the value of the variable.
 *****************************************************/
 
+CONFIG_VARIABLE (Temporal::TimeDomain, preferred_time_domain, "preferred_time_domain", Temporal::BeatTime)
+
 /* IO connection */
 
 CONFIG_VARIABLE (bool, auto_connect_standard_busses, "auto-connect-standard-busses", true)

libs/ardour/session_state.cc

@@ -4588,6 +4588,7 @@ Session::config_changed (std::string p, bool ours)
 		Temporal::TimeDomain td = config.get_default_time_domain ();
 		std::cerr << "Setting time domain\n";
 		set_time_domain (td);
+		Config->set_preferred_time_domain(td);  /* sync the global default time domain to this newly chosen one */
 	}
 
 	set_dirty ();