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.cpp

@@ -28,6 +28,18 @@ textboxclass::textboxclass(void)
 
     print_flags = PR_FONT_LEVEL;
     fill_buttons = false;
+
+    sprites.clear();
+}
+
+void textboxclass::addsprite(int x, int y, int tile, int col)
+{
+    TextboxSprite sprite;
+    sprite.x = x;
+    sprite.y = y;
+    sprite.tile = tile;
+    sprite.col = col;
+    sprites.push_back(sprite);
 }
 
 void textboxclass::centerx(void)