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/Graphics.h

@@ -124,6 +124,8 @@ public:
 
     void addsprite(int x, int y, int tile, int col);
 
+    void setimage(TextboxImage image);
+
     void textboxremove(void);
 
     void textboxremovefast(void);