Add textimage for levelcomplete and gamecomplete

`levelcomplete` and `gamecomplete` were hardcoded using textbox colors
which were offset by 1. This PR fixes that, no longer requiring
slightly-off colors, and instead adding a new property to textboxes
which tell the game to display either level complete or game complete.

desktop_version/src/Script.h

@@ -125,6 +125,7 @@ public:
     bool textlarge;
     int textboxtimer;
     std::vector<TextboxSprite> textbox_sprites;
+    TextboxImage textbox_image;
 
     //Misc
     int i, j, k;