Initial implementation of textbox sprites

This commit adds a system for displaying sprites in textboxes, meant to
replace the hardcoded system in the main game. This does not support
levelcomplete.png and gamecomplete.png yet, which will most likely just
be special cases.

desktop_version/src/Textbox.h

@@ -5,11 +5,21 @@
 #include <string>
 #include <vector>
 
+struct TextboxSprite
+{
+    int x;
+    int y;
+    int col;
+    int tile;
+};
+
 class textboxclass
 {
 public:
     textboxclass(void);
 
+    void addsprite(int x, int y, int tile, int col);
+
     void centerx(void);
 
     void centery(void);
@@ -53,6 +63,8 @@ public:
 
     uint32_t print_flags;
     bool fill_buttons;
+
+    std::vector<TextboxSprite> sprites;
 };
 
 #endif /* TEXTBOX_H */