settings: add startup_slave_screen option

src/settings.h

@@ -187,6 +187,7 @@ struct Configuration
 	size_t now_playing_suffix_length;
 
 	ScreenType startup_screen_type;
+	ScreenType startup_slave_screen_type;
 	std::list<ScreenType> screen_sequence;
 
 	SortMode browser_sort_mode;