LibGUI: Defer loading emoji icons until their first paint event

The EmojiInputDialog re-uses emoji buttons to help with performance as
filters are applied / removed. The downside of pre-creating the buttons
is that it currently takes upwards of 600ms (on my machine) to load all
emoji icons from disk at once. This will only become worse over time as
more emoji are added.

To alleviate this, defer loading the icons until they are needed for
painting (i.e. come into view).

Userland/Libraries/LibGUI/EmojiInputDialog.cpp

@@ -59,6 +59,34 @@ static void resize_bitmap_if_needed(NonnullRefPtr<Gfx::Bitmap>& bitmap)
     }
 }
 
+class EmojiButton final : public Button {
+    C_OBJECT(EmojiButton);
+
+private:
+    explicit EmojiButton(String emoji_icon_path)
+        : Button()
+        , m_emoji_icon_path(move(emoji_icon_path))
+    {
+    }
+
+    virtual void paint_event(PaintEvent& event) override
+    {
+        if (m_first_paint_event) {
+            m_first_paint_event = false;
+
+            auto bitmap = Gfx::Bitmap::try_load_from_file(m_emoji_icon_path).release_value_but_fixme_should_propagate_errors();
+            resize_bitmap_if_needed(bitmap);
+
+            set_icon(move(bitmap));
+        }
+
+        Button::paint_event(event);
+    }
+
+    bool m_first_paint_event { true };
+    String m_emoji_icon_path;
+};
+
 EmojiInputDialog::EmojiInputDialog(Window* parent_window)
     : Dialog(parent_window)
     , m_category_action_group(make<ActionGroup>())
@@ -158,11 +186,7 @@ auto EmojiInputDialog::supported_emoji() -> Vector<Emoji>
             emoji->display_order = NumericLimits<u32>::max();
         }
 
-        auto bitmap = Gfx::Bitmap::try_load_from_file(filename).release_value_but_fixme_should_propagate_errors();
-        resize_bitmap_if_needed(bitmap);
-
-        auto button = Button::construct();
-        button->set_icon(bitmap);
+        auto button = EmojiButton::construct(move(filename));
         button->set_fixed_size(button_size, button_size);
         button->set_button_style(Gfx::ButtonStyle::Coolbar);
         button->on_click = [this, text = builder.to_string()](auto) {