Fix up the backend so it has sane names like get_sprite and get_icon, then implemented a guecs::Icon for icons. Those don't scale and typically have other properties different from a Sprite.

include/guecs/sfml/textures.hpp

@@ -11,6 +11,7 @@ namespace textures {
   struct SpriteTexture {
     std::shared_ptr<sf::Sprite> sprite = nullptr;
     std::shared_ptr<sf::Texture> texture = nullptr;
+    sf::Vector2i frame_size;
   };
 
   struct TextureManager {