add newline feature to the text node, add about window with credits, add about menu with submenus

src/app.h

@@ -139,6 +139,8 @@ public:
     void init_menu_edit();
     void init_menu_layer();
     void init_menu_timelapse();
+    void init_menu_about();
+    void dialog_about();
     void dialog_newdoc();
     void dialog_save();
     void dialog_save_ver();