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/Textbox.h

@@ -137,6 +137,9 @@ public:
     std::vector<TextboxSprite> sprites;
     TextboxImage image;
 
+    bool force_outline;
+    bool outline;
+
     TextboxCrewmatePosition crewmate_position;
     TextboxOriginalContext original;
     TextboxSpacing spacing;