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/Textbox.h

@@ -13,6 +13,13 @@ struct TextboxSprite
     int tile;
 };
 
+enum TextboxImage
+{
+    TEXTIMAGE_NONE,
+    TEXTIMAGE_LEVELCOMPLETE,
+    TEXTIMAGE_GAMECOMPLETE
+};
+
 class textboxclass
 {
 public:
@@ -20,6 +27,8 @@ public:
 
     void addsprite(int x, int y, int tile, int col);
 
+    void setimage(TextboxImage image);
+
     void centerx(void);
 
     void centery(void);
@@ -65,6 +74,7 @@ public:
     bool fill_buttons;
 
     std::vector<TextboxSprite> sprites;
+    TextboxImage image;
 };
 
 #endif /* TEXTBOX_H */