LibDraw: Store emojis in a HashMap<u32, RefPtr<GraphicsBitmap>>

Get rid of the dedicated Emoji class to make it easier to store a null
value signifying a failed lookup.

This allows us to remember failed lookups, making subsequent failures
for the same codepoint much faster. :^)

Libraries/LibDraw/Emoji.h

@@ -1,19 +1,10 @@
 #pragma once
 
 #include <AK/Types.h>
-#include <AK/NonnullRefPtr.h>
 
 class GraphicsBitmap;
 
 class Emoji {
- public:
-    ~Emoji() {}
-
-    static const Emoji* emoji_for_codepoint(u32 codepoint);
-    const GraphicsBitmap& bitmap() const { return m_bitmap; }
-
-private:
-    explicit Emoji(NonnullRefPtr<GraphicsBitmap>);
-
-    NonnullRefPtr<GraphicsBitmap> m_bitmap;
+public:
+    static const GraphicsBitmap* emoji_for_codepoint(u32 codepoint);
 };