create and manage a new config parameter that defines where LTC originates (still some tweaks to be done here and there)

git-svn-id: svn://localhost/ardour2/branches/3.0@13280 d708f5d6-7413-0410-9779-e7cbd77b26cf

gtk2_ardour/rc_option_editor.h

@@ -45,6 +45,7 @@ private:
 	ComboOption<ARDOUR::SyncSource>* _sync_source;
         BoolOption* _sync_framerate;
         BoolOption* _sync_genlock;
+        ComboStringOption* _ltc_port;
 
         PBD::ScopedConnection parameter_change_connection;
 };