fix [on_undo] in menu items and custom_command

also fixes menu items beeing marked as undoabel even if they used the
synced rng.

src/actions/undo.cpp

@@ -107,6 +107,8 @@ undo_action_base * undo_list::create_action(const config & cfg)
 
 	else if ( str == "update_shroud" )
 		res =  new undo::update_shroud_action();
+	else if ( str == "dummy" )
+		res =  new undo_dummy_action(cfg);
 	else
 	{
 		// Unrecognized type.

src/actions/undo_action.hpp

@@ -101,6 +101,7 @@ namespace actions {
 		/// Undoes this action.
 		virtual bool undo(int)
 		{
+			execute_undo_umc_wml();
 			return true;
 		}
 	};

src/synced_commands.cpp

@@ -336,7 +336,7 @@ SYNCED_COMMAND_HANDLER_FUNCTION(fire_event, child,  use_undo, /*show*/, /*error_
 
 	// Not clearing the undo stack here causes OOS because we added an entry to the replay but no entry to the undo stack.
 	if(use_undo) {
-		if(!undoable) {
+		if(!undoable || !synced_context::can_undo()) {
 			resources::undo_stack->clear();
 		} else {
 			resources::undo_stack->add_dummy();
@@ -345,10 +345,17 @@ SYNCED_COMMAND_HANDLER_FUNCTION(fire_event, child,  use_undo, /*show*/, /*error_
 	return true;
 }
 
-SYNCED_COMMAND_HANDLER_FUNCTION(custom_command, child,  /*use_undo*/, /*show*/, /*error_handler*/)
+SYNCED_COMMAND_HANDLER_FUNCTION(custom_command, child,  use_undo, /*show*/, /*error_handler*/)
 {
 	assert(resources::lua_kernel);
 	resources::lua_kernel->custom_command(child["name"], child.child_or_empty("data"));
+	if(use_undo) {
+		if(!synced_context::can_undo()) {
+			resources::undo_stack->clear();
+		} else {
+			resources::undo_stack->add_dummy();
+		}
+	}
 	return true;
 }