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/Graphics.h

@@ -137,6 +137,8 @@ public:
 
     void setimage(TextboxImage image);
 
+    void textboxoutline(bool enabled);
+
     void textboxindex(int index);
 
     void textboxremove(void);