Add textoutline(on/off/default)

This commit adds a new scripting command for textbox visual control,
where you're able to force the transparent textbox's outline on or off.

desktop_version/src/Script.h

@@ -123,6 +123,8 @@ public:
     std::vector<TextboxSprite> textbox_sprites;
     TextboxImage textbox_image;
     bool textbox_forcepos;
+    bool textbox_force_outline;
+    bool textbox_outline;
 
     //Misc
     int i, j, k;